message: core: fix autopilot detection
  This was obviously very wrong.
diff: @@ -191,9 +191,9 @@ void SystemImpl::process_heartbeat(const mavlink_message_t& message)
  }
  if (heartbeat.autopilot == MAV_AUTOPILOT_PX4) {
  - _autopilot = Autopilot::ArduPilot;
  - } else if (heartbeat.autopilot == MAV_AUTOPILOT_ARDUPILOTMEGA) {
  _autopilot = Autopilot::Px4;
  + } else if (heartbeat.autopilot == MAV_AUTOPILOT_ARDUPILOTMEGA) {
  + _autopilot = Autopilot::ArduPilot;
  }
  set_connected();
  