Made USP-API included flexible

Author: NicoHood

Diff for: hardware/arduino/avr/cores/arduino/USBAPI.h

@@ -67,6 +67,7 @@ class USBDevice_
 	bool wakeupHost(); // returns false, when wakeup cannot be processed
 };
 extern USBDevice_ USBDevice;
+void setupUSB();
 
 //================================================================================
 //================================================================================

Diff for: hardware/arduino/avr/cores/arduino/USBCore.cpp

@@ -828,4 +828,8 @@ bool USBDevice_::wakeupHost()
 	return false;
 }
 
+void setupUSB() {
+	USBDevice.attach();
+}
+
 #endif /* if defined(USBCON) */

Diff for: hardware/arduino/avr/cores/arduino/main.cpp

@@ -36,9 +36,7 @@ int main(void)
 
 	initVariant();
 
-#if defined(USBCON)
-	USBDevice.attach();
-#endif
+	setupUSB();
 
 	setup();