Extend RX/TX pins for up to 8 extruders

Marlin/src/pins/linux/pins_RAMPS_LINUX.h

   #ifndef E4_SERIAL_RX_PIN
     #define E4_SERIAL_RX_PIN -1
   #endif
+  #ifndef E5_SERIAL_TX_PIN
+    #define E5_SERIAL_TX_PIN -1
+  #endif
+  #ifndef E5_SERIAL_RX_PIN
+    #define E5_SERIAL_RX_PIN -1
+  #endif
+  #ifndef E6_SERIAL_TX_PIN
+    #define E6_SERIAL_TX_PIN -1
+  #endif
+  #ifndef E6_SERIAL_RX_PIN
+    #define E6_SERIAL_RX_PIN -1
+  #endif
+  #ifndef E7_SERIAL_TX_PIN
+    #define E7_SERIAL_TX_PIN -1
+  #endif
+  #ifndef E7_SERIAL_RX_PIN
+    #define E7_SERIAL_RX_PIN -1
+  #endif
 #endif
 
 //////////////////////////

Marlin/src/pins/ramps/pins_RAMPS.h

   #ifndef E4_SERIAL_RX_PIN
     #define E4_SERIAL_RX_PIN -1
   #endif
+  #ifndef E5_SERIAL_TX_PIN
+    #define E5_SERIAL_TX_PIN -1
+  #endif
+  #ifndef E5_SERIAL_RX_PIN
+    #define E5_SERIAL_RX_PIN -1
+  #endif
+  #ifndef E6_SERIAL_TX_PIN
+    #define E6_SERIAL_TX_PIN -1
+  #endif
+  #ifndef E6_SERIAL_RX_PIN
+    #define E6_SERIAL_RX_PIN -1
+  #endif
+  #ifndef E7_SERIAL_TX_PIN
+    #define E7_SERIAL_TX_PIN -1
+  #endif
+  #ifndef E7_SERIAL_RX_PIN
+    #define E7_SERIAL_RX_PIN -1
+  #endif
 #endif
 
 //

Marlin/src/pins/ramps/pins_TT_OSCAR.h

   #define E3_SERIAL_RX_PIN -1
   #define E4_SERIAL_TX_PIN -1
   #define E4_SERIAL_RX_PIN -1
+  #define E5_SERIAL_RX_PIN -1
+  #define E6_SERIAL_RX_PIN -1
+  #define E7_SERIAL_RX_PIN -1
 #endif
 
 //

Marlin/src/pins/ramps/pins_Z_BOLT_X_SERIES.h

   #ifndef E4_SERIAL_RX_PIN
     #define E4_SERIAL_RX_PIN -1
   #endif
+  #ifndef E5_SERIAL_TX_PIN
+    #define E5_SERIAL_TX_PIN -1
+  #endif
+  #ifndef E5_SERIAL_RX_PIN
+    #define E5_SERIAL_RX_PIN -1
+  #endif
+  #ifndef E6_SERIAL_TX_PIN
+    #define E6_SERIAL_TX_PIN -1
+  #endif
+  #ifndef E6_SERIAL_RX_PIN
+    #define E6_SERIAL_RX_PIN -1
+  #endif
+  #ifndef E7_SERIAL_TX_PIN
+    #define E7_SERIAL_TX_PIN -1
+  #endif
+  #ifndef E7_SERIAL_RX_PIN
+    #define E7_SERIAL_RX_PIN -1
+  #endif
 #endif