DFU: towards a more complete implementation

* we now actually route the EP0 control requests in DFU mode to the
  correct handler (weak linker symbols are tricky)
* we now actually call code to read/write data from/to RAM/FLASH

firmware/atmel_softpack_libraries/usb/device/dfu/dfu_runtime.c

@@ -187,3 +187,9 @@ void DFURT_SwitchToDFU(void)
 
 	toDFU();
 }
+
+void USBDCallbacks_RequestReceived(const USBGenericRequest *request)
+{
+	/* FIXME: integration with CCID control point reqeusts */
+	USBDFU_Runtime_RequestHandler(request);
+}