message: core: notify when fake system becomes real system
  This makes sure we notify the user when a system actually connects after
  it has been created in anticipation that it will eventually exist.
diff: @@ -307,6 +307,10 @@ void MavsdkImpl::receive_message(mavlink_message_t& message, Connection* connect
  << " Comp ID: " << static_cast<int>(message.compid);
  _systems[0].first = message.sysid;
  _systems[0].second->system_impl()->set_system_id(message.sysid);
  +
  + // Even though the fake system was already discovered, we can now
  + // send a notification, now that it seems to really actually exist.
  + notify_on_discover();
  }
  bool found_system = false;
  