Improve DFU procedure :

- correctly write all bytes to flash
 - check CRC
 - Fix bug in notification : they cannot be sent from the control point handler (because it seems you cannot send a notification and a write acknowledge at the same time) using a timer (quick'n'dirty implementation to be improved)
 - Improve dfu screen
 - Reset if dfu image is correctly copied into flash and crc is ok.

src/SystemTask/SystemTask.cpp

@@ -118,6 +118,7 @@ void SystemTask::Work() {
           break;
         case Messages::BleFirmwareUpdateFinished:
           displayApp->PushMessage(Pinetime::Applications::DisplayApp::Messages::BleFirmwareUpdateFinished);
+          NVIC_SystemReset();
           break;
         default: break;
       }