Rename BTENABLED to BLUETOOTH

- For some reason it’s easy to forget what BT stands for, but it
doesn’t need an abbreviation.

Marlin/Marlin.h

@@ -51,11 +51,11 @@ typedef unsigned long millis_t;
 #include "WString.h"
 
 #ifdef USBCON
-  #if ENABLED(BTENABLED)
+  #if ENABLED(BLUETOOTH)
     #define MYSERIAL bt
   #else
     #define MYSERIAL Serial
-  #endif // BTENABLED
+  #endif // BLUETOOTH
 #else
   #define MYSERIAL MSerial
 #endif