message: mavlink_commands: fallthrough attribute for GCC 7
  GCC 7 warns of implicit fallthroughs unless you add
  __attribute__((fallthrough)) or [[fallthrough]].
diff: @@ -157,11 +157,11 @@ void MavlinkCommands::receive_command_ack(mavlink_message_t message)
  case MAV_RESULT_DENIED:
  Debug() << "command denied (" << work.mavlink_command << ").";
  - // no break
  + [[fallthrough]]; // no break
  case MAV_RESULT_UNSUPPORTED:
  Debug() << "command unsupported (" << work.mavlink_command << ").";
  - // no break
  + [[fallthrough]]; // no break
  case MAV_RESULT_FAILED:
  Debug() << "command failed (" << work.mavlink_command << ").";
  