message: core: don't warn on first discovery
  There is no point in having a warning when first changing from the
  initial MAV_TYPE_GENERIC, hence we ignore that.
diff: @@ -220,7 +220,8 @@ void SystemImpl::process_heartbeat(const mavlink_message_t& message)
  LogErr() << "type received in HEARTBEAT was not recognized";
  } else {
  const auto new_vehicle_type = static_cast<MAV_TYPE>(heartbeat.type);
  - if (heartbeat.autopilot != MAV_AUTOPILOT_INVALID && _vehicle_type != new_vehicle_type) {
  + if (heartbeat.autopilot != MAV_AUTOPILOT_INVALID && _vehicle_type != new_vehicle_type &&
  + _vehicle_type != MAV_TYPE_GENERIC) {
  LogWarn() << "Vehicle type changed (new type: " << static_cast<unsigned>(heartbeat.type)
  << ", old type: " << static_cast<unsigned>(_vehicle_type) << ")";
  _vehicle_type = new_vehicle_type;
  