APM: support DO_SET_SERVO command

ArduPlane/GCS_Mavlink.pde

@@ -1105,6 +1105,10 @@ void GCS_MAVLINK::handleMessage(mavlink_message_t* msg)
             }
             break;
 
+        case MAV_CMD_DO_SET_SERVO:
+            APM_RC.OutputCh(packet.param1 - 1, packet.param2);
+            result = MAV_RESULT_ACCEPTED;
+            break;
 
         default:
             result = MAV_RESULT_UNSUPPORTED;