added metro nrf52840 revA

clean up makefile

src/usb/usb.c

@@ -79,6 +79,8 @@ void usb_init(bool cdc_only)
 
   _inited = true;
 
+  NVIC_SetPriority(USBD_IRQn, 2);
+
   // USB power may already be ready at this time -> no event generated
   // We need to invoke the handler based on the status initially
   uint32_t usb_reg;