Use lowercase names for serial instances

Marlin/Marlin.h

@@ -52,12 +52,12 @@ typedef unsigned long millis_t;
 
 #ifdef USBCON
   #if ENABLED(BLUETOOTH)
-    #define MYSERIAL bt
+    #define MYSERIAL bluetoothSerial
   #else
     #define MYSERIAL Serial
   #endif // BLUETOOTH
 #else
-  #define MYSERIAL MSerial
+  #define MYSERIAL customizedSerial
 #endif
 
 #define SERIAL_CHAR(x) MYSERIAL.write(x)

Marlin/MarlinSerial.cpp

@@ -281,12 +281,12 @@ void MarlinSerial::printFloat(double number, uint8_t digits) {
 // Preinstantiate Objects //////////////////////////////////////////////////////
 
 
-MarlinSerial MSerial;
+MarlinSerial customizedSerial;
 
 #endif // whole file
 #endif // !USBCON
 
 // For AT90USB targets use the UART for BT interfacing
 #if defined(USBCON) && ENABLED(BLUETOOTH)
-  HardwareSerial bt;
+  HardwareSerial bluetoothSerial;
 #endif

Marlin/MarlinSerial.h

@@ -149,12 +149,12 @@ class MarlinSerial { //: public Stream
     void println(void);
 };
 
-extern MarlinSerial MSerial;
+extern MarlinSerial customizedSerial;
 #endif // !USBCON
 
-// Use the UART for BT in AT90USB configurations
+// Use the UART for Bluetooth in AT90USB configurations
 #if defined(USBCON) && ENABLED(BLUETOOTH)
-  extern HardwareSerial bt;
+  extern HardwareSerial bluetoothSerial;
 #endif
 
 #endif

Marlin/stepper.cpp

@@ -655,7 +655,7 @@ ISR(TIMER1_COMPA_vect) {
     // Take multiple steps per interrupt (For high speed moves)
     for (int8_t i = 0; i < step_loops; i++) {
       #ifndef USBCON
-        MSerial.checkRx(); // Check for serial chars.
+        customizedSerial.checkRx(); // Check for serial chars.
       #endif
 
       #if ENABLED(ADVANCE)