add weak setupUSB() hook

override this function to insert additional USB endpoints

cores/arduino/main.cpp

@@ -27,6 +27,9 @@ int atexit(void (* /*func*/ )()) { return 0; }
 void initVariant() __attribute__((weak));
 void initVariant() { }
 
+void setupUSB() __attribute__((weak));
+void setupUSB() { }
+
 int main(void)
 {
 	init();
@@ -34,6 +37,7 @@ int main(void)
 	initVariant();
 
 #if defined(USBCON)
+	setupUSB();
 	USBDevice.attach();
 #endif