🧑‍💻 Clarify MSerialUSB, drop HardwareSerial workaround

Marlin/src/HAL/AVR/HAL.h

 #ifdef USBCON
   #include <HardwareSerial.h>
 #else
-  #define HardwareSerial_h // Hack to prevent HardwareSerial.h header inclusion
   #include "MarlinSerial.h"
 #endif
 

Marlin/src/HAL/STM32/HAL.cpp

 #include "../shared/Delay.h"
 
 #ifdef USBCON
-  DefaultSerial1 MSerial0(false, SerialUSB);
+  DefaultSerial1 MSerialUSB(false, SerialUSB);
 #endif
 
 #if ENABLED(SRAM_EEPROM_EMULATION)

Marlin/src/HAL/STM32/HAL.h

   #include <USBSerial.h>
   #include "../../core/serial_hook.h"
   typedef ForwardSerial1Class< decltype(SerialUSB) > DefaultSerial1;
-  extern DefaultSerial1 MSerial0;
+  extern DefaultSerial1 MSerialUSB;
 #endif
 
 #define _MSERIAL(X) MSerial##X
 #define MSERIAL(X) _MSERIAL(X)
 
-#if SERIAL_PORT == -1
-  #define MYSERIAL1 MSerial0
-#elif WITHIN(SERIAL_PORT, 1, 6)
+#if WITHIN(SERIAL_PORT, 1, 6)
   #define MYSERIAL1 MSERIAL(SERIAL_PORT)
+#elif !defined(USBCON)
+  #error "SERIAL_PORT must be from 1 to 6."
+#elif SERIAL_PORT == -1
+  #define MYSERIAL1 MSerialUSB
 #else
-  #error "SERIAL_PORT must be from 1 to 6. You can also use -1 if the board supports Native USB."
+  #error "SERIAL_PORT must be from 1 to 6, or -1 for Native USB."
 #endif
 
 #ifdef SERIAL_PORT_2
-  #if SERIAL_PORT_2 == -1
-    #define MYSERIAL2 MSerial0
-  #elif WITHIN(SERIAL_PORT_2, 1, 6)
+  #if WITHIN(SERIAL_PORT_2, 1, 6)
     #define MYSERIAL2 MSERIAL(SERIAL_PORT_2)
+  #elif !defined(USBCON)
+    #error "SERIAL_PORT must be from 1 to 6."
+  #elif SERIAL_PORT_2 == -1
+    #define MYSERIAL2 MSerialUSB
   #else
-    #error "SERIAL_PORT_2 must be from 1 to 6. You can also use -1 if the board supports Native USB."
+    #error "SERIAL_PORT_2 must be from 1 to 6, or -1 for Native USB."
   #endif
 #endif
 
 #ifdef SERIAL_PORT_3
-  #if SERIAL_PORT_3 == -1
-    #define MYSERIAL3 MSerial0
-  #elif WITHIN(SERIAL_PORT_3, 1, 6)
+  #if WITHIN(SERIAL_PORT_3, 1, 6)
     #define MYSERIAL3 MSERIAL(SERIAL_PORT_3)
+  #elif !defined(USBCON)
+    #error "SERIAL_PORT must be from 1 to 6."
+  #elif SERIAL_PORT_3 == -1
+    #define MYSERIAL3 MSerialUSB
   #else
-    #error "SERIAL_PORT_3 must be from 1 to 6. You can also use -1 if the board supports Native USB."
+    #error "SERIAL_PORT_3 must be from 1 to 6, or -1 for Native USB."
   #endif
 #endif
 
 #ifdef MMU2_SERIAL_PORT
-  #if MMU2_SERIAL_PORT == -1
-    #define MMU2_SERIAL MSerial0
-  #elif WITHIN(MMU2_SERIAL_PORT, 1, 6)
+  #if WITHIN(MMU2_SERIAL_PORT, 1, 6)
     #define MMU2_SERIAL MSERIAL(MMU2_SERIAL_PORT)
+  #elif !defined(USBCON)
+    #error "SERIAL_PORT must be from 1 to 6."
+  #elif MMU2_SERIAL_PORT == -1
+    #define MMU2_SERIAL MSerialUSB
   #else
-    #error "MMU2_SERIAL_PORT must be from 1 to 6. You can also use -1 if the board supports Native USB."
+    #error "MMU2_SERIAL_PORT must be from 1 to 6, or -1 for Native USB."
   #endif
 #endif
 
 #ifdef LCD_SERIAL_PORT
-  #if LCD_SERIAL_PORT == -1
-    #define LCD_SERIAL MSerial0
-  #elif WITHIN(LCD_SERIAL_PORT, 1, 6)
+  #if WITHIN(LCD_SERIAL_PORT, 1, 6)
     #define LCD_SERIAL MSERIAL(LCD_SERIAL_PORT)
+  #elif !defined(USBCON)
+    #error "SERIAL_PORT must be from 1 to 6."
+  #elif LCD_SERIAL_PORT == -1
+    #define LCD_SERIAL MSerialUSB
   #else
-    #error "LCD_SERIAL_PORT must be from 1 to 6. You can also use -1 if the board supports Native USB."
+    #error "LCD_SERIAL_PORT must be from 1 to 6, or -1 for Native USB."
   #endif
   #if HAS_DGUS_LCD
     #define SERIAL_GET_TX_BUFFER_FREE() LCD_SERIAL.availableForWrite()

Marlin/src/core/serial_hook.h

 // Build the actual serial object depending on current configuration
 #define Serial1Class TERN(SERIAL_RUNTIME_HOOK, RuntimeSerial, BaseSerial)
 #define ForwardSerial1Class TERN(SERIAL_RUNTIME_HOOK, RuntimeSerial, ForwardSerial)
-#ifdef HAS_MULTI_SERIAL
+#if HAS_MULTI_SERIAL
   #define Serial2Class ConditionalSerial
   #if NUM_SERIAL >= 3
     #define Serial3Class ConditionalSerial