message: Immediately call new_system_callback if a system is already present
diff: @@ -456,6 +456,10 @@ void MavsdkImpl::subscribe_on_new_system(Mavsdk::NewSystemCallback callback)
  {
  std::lock_guard<std::mutex> lock(_new_system_callback_mutex);
  _new_system_callback = callback;
  +
  + if (_new_system_callback != nullptr && systems().size() > 0) {
  + _new_system_callback();
  + }
  }
  void MavsdkImpl::register_on_discover(const Mavsdk::event_callback_t callback)
  