Add HAS_MULTI_SERIAL conditional

Marlin/src/MarlinCore.cpp

     MYSERIAL0.begin(BAUDRATE);
     uint32_t serial_connect_timeout = millis() + 1000UL;
     while (!MYSERIAL0 && PENDING(millis(), serial_connect_timeout)) { /*nada*/ }
-    #if NUM_SERIAL > 1
+    #if HAS_MULTI_SERIAL
       MYSERIAL1.begin(BAUDRATE);
       serial_connect_timeout = millis() + 1000UL;
       while (!MYSERIAL1 && PENDING(millis(), serial_connect_timeout)) { /*nada*/ }

Marlin/src/core/serial.cpp

 static PGMSTR(errormagic, "Error:");
 static PGMSTR(echomagic, "echo:");
 
-#if NUM_SERIAL > 1
+#if HAS_MULTI_SERIAL
   int8_t serial_port_index = 0;
 #endif
 

Marlin/src/core/serial.h

 #define DEBUGGING(F) (marlin_debug_flags & (MARLIN_DEBUG_## F))
 
 #define SERIAL_BOTH 0x7F
-#if NUM_SERIAL > 1
+#if HAS_MULTI_SERIAL
   extern int8_t serial_port_index;
   #define _PORT_REDIRECT(n,p)   REMEMBER(n,serial_port_index,p)
   #define _PORT_RESTORE(n)      RESTORE(n)

Marlin/src/feature/binary_protocol.h

 inline bool bs_serial_data_available(const uint8_t index) {
   switch (index) {
     case 0: return MYSERIAL0.available();
-    #if NUM_SERIAL > 1
+    #if HAS_MULTI_SERIAL
       case 1: return MYSERIAL1.available();
     #endif
   }
 inline int bs_read_serial(const uint8_t index) {
   switch (index) {
     case 0: return MYSERIAL0.read();
-    #if NUM_SERIAL > 1
+    #if HAS_MULTI_SERIAL
       case 1: return MYSERIAL1.read();
     #endif
   }

Marlin/src/gcode/config/M575.cpp

       if (set0) {
         SERIAL_ECHO_START();
         SERIAL_ECHOLNPAIR(" Serial "
-          #if NUM_SERIAL > 1
+          #if HAS_MULTI_SERIAL
             , '0',
           #else
             "0"
           " baud rate set to ", baud
         );
       }
-      #if NUM_SERIAL > 1
+      #if HAS_MULTI_SERIAL
         const bool set1 = (port == -99 || port == 1);
         if (set1) {
           SERIAL_ECHO_START();
 
       if (set0) { MYSERIAL0.end(); MYSERIAL0.begin(baud); }
 
-      #if NUM_SERIAL > 1
+      #if HAS_MULTI_SERIAL
         if (set1) { MYSERIAL1.end(); MYSERIAL1.begin(baud); }
       #endif
 

Marlin/src/gcode/host/M118.cpp

  */
 void GcodeSuite::M118() {
   bool hasE = false, hasA = false;
-  #if NUM_SERIAL > 1
+  #if HAS_MULTI_SERIAL
     int8_t port = -1; // Assume no redirect
   #endif
   char *p = parser.string_arg;
     switch (p[0]) {
       case 'A': hasA = true; break;
       case 'E': hasE = true; break;
-      #if NUM_SERIAL > 1
+      #if HAS_MULTI_SERIAL
         case 'P': port = p[1] - '0'; break;
       #endif
     }
     while (*p == ' ') ++p;
   }
 
-  #if NUM_SERIAL > 1
+  #if HAS_MULTI_SERIAL
     const int8_t old_serial = serial_port_index;
     if (WITHIN(port, 0, NUM_SERIAL))
       serial_port_index = (
   if (hasA) SERIAL_ECHOPGM("// ");
   SERIAL_ECHOLN(p);
 
-  #if NUM_SERIAL > 1
-    serial_port_index = old_serial;
-  #endif
+  TERN_(HAS_MULTI_SERIAL, serial_port_index = old_serial);
 }

Marlin/src/gcode/parser.cpp

 
 #include "../MarlinCore.h"
 
-#if NUM_SERIAL > 1
+#if HAS_MULTI_SERIAL
   #include "queue.h"
 #endif
 

Marlin/src/gcode/queue.cpp

 /*
  * The port that the command was received on
  */
-#if NUM_SERIAL > 1
+#if HAS_MULTI_SERIAL
   int16_t GCodeQueue::port[BUFSIZE];
 #endif
 
  * Once a new command is in the ring buffer, call this to commit it
  */
 void GCodeQueue::_commit_command(bool say_ok
-  #if NUM_SERIAL > 1
+  #if HAS_MULTI_SERIAL
     , int16_t p/*=-1*/
   #endif
 ) {
   send_ok[index_w] = say_ok;
-  #if NUM_SERIAL > 1
-    port[index_w] = p;
-  #endif
+  TERN_(HAS_MULTI_SERIAL, port[index_w] = p);
   TERN_(POWER_LOSS_RECOVERY, recovery.commit_sdpos(index_w));
   if (++index_w >= BUFSIZE) index_w = 0;
   length++;
  * Return false for a full buffer, or if the 'command' is a comment.
  */
 bool GCodeQueue::_enqueue(const char* cmd, bool say_ok/*=false*/
-  #if NUM_SERIAL > 1
+  #if HAS_MULTI_SERIAL
     , int16_t pn/*=-1*/
   #endif
 ) {
   if (*cmd == ';' || length >= BUFSIZE) return false;
   strcpy(command_buffer[index_w], cmd);
   _commit_command(say_ok
-    #if NUM_SERIAL > 1
+    #if HAS_MULTI_SERIAL
       , pn
     #endif
   );
  *   B<int>  Block queue space remaining
  */
 void GCodeQueue::ok_to_send() {
-  #if NUM_SERIAL > 1
+  #if HAS_MULTI_SERIAL
     const int16_t pn = command_port();
     if (pn < 0) return;
     PORT_REDIRECT(pn);                    // Reply to the serial port that sent the command
  */
 void GCodeQueue::flush_and_request_resend() {
   const int16_t pn = command_port();
-  #if NUM_SERIAL > 1
+  #if HAS_MULTI_SERIAL
     if (pn < 0) return;
     PORT_REDIRECT(pn);                    // Reply to the serial port that sent the command
   #endif
   SERIAL_FLUSH();
   SERIAL_ECHOPGM(STR_RESEND);
-  
-  SERIAL_ECHOLN(last_N[pn] + 1);  
+  SERIAL_ECHOLN(last_N[pn] + 1);
   ok_to_send();
 }
 
 inline bool serial_data_available() {
-  return false
-    || MYSERIAL0.available()
-    #if NUM_SERIAL > 1
-      || MYSERIAL1.available()
-    #endif
-  ;
+  return MYSERIAL0.available() || TERN0(HAS_MULTI_SERIAL, MYSERIAL1.available());
 }
 
 inline int read_serial(const uint8_t index) {
   switch (index) {
     case 0: return MYSERIAL0.read();
-    #if NUM_SERIAL > 1
+    #if HAS_MULTI_SERIAL
       case 1: return MYSERIAL1.read();
     #endif
     default: return -1;
 
         // Add the command to the queue
         _enqueue(serial_line_buffer[i], true
-          #if NUM_SERIAL > 1
+          #if HAS_MULTI_SERIAL
             , i
           #endif
         );

Marlin/src/gcode/queue.h

   /**
    * The port that the command was received on
    */
-  #if NUM_SERIAL > 1
+  #if HAS_MULTI_SERIAL
     static int16_t port[BUFSIZE];
   #endif
 
   static int16_t command_port() {
-    return (0
-      #if NUM_SERIAL > 1
-        + port[index_r]
-      #endif
-    );
+    return TERN0(HAS_MULTI_SERIAL, port[index_r]);
   }
 
   GCodeQueue();
   #endif
 
   static void _commit_command(bool say_ok
-    #if NUM_SERIAL > 1
+    #if HAS_MULTI_SERIAL
       , int16_t p=-1
     #endif
   );
 
   static bool _enqueue(const char* cmd, bool say_ok=false
-    #if NUM_SERIAL > 1
+    #if HAS_MULTI_SERIAL
       , int16_t p=-1
     #endif
   );

Marlin/src/gcode/sd/M28_M29.cpp

 #include "../gcode.h"
 #include "../../sd/cardreader.h"
 
-#if NUM_SERIAL > 1
+#if HAS_MULTI_SERIAL
   #include "../queue.h"
 #endif
 
     // Binary transfer mode
     if ((card.flag.binary_mode = binary_mode)) {
       SERIAL_ECHO_MSG("Switching to Binary Protocol");
-      #if NUM_SERIAL > 1
-        card.transfer_port_index = queue.port[queue.index_r];
-      #endif
+      TERN_(HAS_MULTI_SERIAL, card.transfer_port_index = queue.port[queue.index_r]);
     }
     else
       card.openFileWrite(p);

Marlin/src/inc/Conditionals_post.h

 
 #if !NUM_SERIAL
   #undef BAUD_RATE_GCODE
+#elif NUM_SERIAL > 1
+  #define HAS_MULTI_SERIAL 1
 #endif

Marlin/src/sd/cardreader.cpp

 char CardReader::filename[FILENAME_LENGTH], CardReader::longFilename[LONG_FILENAME_LENGTH];
 int8_t CardReader::autostart_index;
 
-#if ENABLED(BINARY_FILE_TRANSFER) && NUM_SERIAL > 1
+#if BOTH(HAS_MULTI_SERIAL, BINARY_FILE_TRANSFER)
   int8_t CardReader::transfer_port_index;
 #endif
 
 #if ENABLED(AUTO_REPORT_SD_STATUS)
   uint8_t CardReader::auto_report_sd_interval = 0;
   millis_t CardReader::next_sd_report_ms;
-  #if NUM_SERIAL > 1
+  #if HAS_MULTI_SERIAL
     int8_t CardReader::auto_report_port;
   #endif
 

Marlin/src/sd/cardreader.h

 
   // Fast! binary file transfer
   #if ENABLED(BINARY_FILE_TRANSFER)
-    #if NUM_SERIAL > 1
+    #if HAS_MULTI_SERIAL
       static int8_t transfer_port_index;
     #else
       static constexpr int8_t transfer_port_index = 0;
   #if ENABLED(AUTO_REPORT_SD_STATUS)
     static void auto_report_sd_status();
     static inline void set_auto_report_interval(uint8_t v) {
-      #if NUM_SERIAL > 1
-        auto_report_port = serial_port_index;
-      #endif
+      TERN_(HAS_MULTI_SERIAL, auto_report_port = serial_port_index);
       NOMORE(v, 60);
       auto_report_sd_interval = v;
       next_sd_report_ms = millis() + 1000UL * v;
   #if ENABLED(AUTO_REPORT_SD_STATUS)
     static uint8_t auto_report_sd_interval;
     static millis_t next_sd_report_ms;
-    #if NUM_SERIAL > 1
+    #if HAS_MULTI_SERIAL
       static int8_t auto_report_port;
     #endif
   #endif