message: Correct Autopilot type of Self (GCS)
  Currently GCS type (in Heartbeat) is MAV_AUTOPILOT_GENERIC (its a type for autopilot!).
  However, MAVLink documentation recommends to use MAV_AUTOPILOT_INVALID for GCS.
diff: @@ -285,8 +285,9 @@ void Device::device_thread(Device *self)
  void Device::send_heartbeat(Device *self)
  {
  mavlink_message_t message;
  + // Self is GCS (its not autopilot!); hence MAV_AUTOPILOT_INVALID.
  mavlink_msg_heartbeat_pack(_own_system_id, _own_component_id, &message,
  - MAV_TYPE_GCS, MAV_AUTOPILOT_GENERIC, 0, 0, 0);
  + MAV_TYPE_GCS, MAV_AUTOPILOT_INVALID, 0, 0, 0);
  self->send_message(message);
  }
  