Fixed the AdaDFU does not re-appear after a failed OTA (adafruit#259)…

… problem, which now provides safer OTA updates

src/main.c

@@ -224,6 +224,7 @@ int main(void)
     bootloader_app_start();
   }
 
+  NRF_POWER->GPREGRET = 0xA8; // No application was loaded, reset the system with the OTA DFU update
   NVIC_SystemReset();
 }