Rely on a definition rather than the magic key

To enable this function you could use
arduino/arduino-builder#15

Author: NicoHood

hardware/arduino/avr/cores/arduino/main.cpp

@@ -29,7 +29,7 @@ void initVariant() { }
 
 void setupUSB() __attribute__((weak));
 void setupUSB() {
-#if defined(USBCON) && (MAGIC_KEY_POS != (RAMEND-1))
+#if defined(USBCON) && defined(DYNAMIC_USB_CORE_INCLUDE))
 	USBDevice.attach();
 #endif
 }