Plane: add support for 4th mavlink channel

ArduPlane/Parameters.h

@@ -137,6 +137,7 @@ public:
         k_param_land_disarm_delay,
         k_param_glide_slope_threshold,
         k_param_rudder_only,
+        k_param_gcs3,            // 93
 
         // 100: Arming parameters
         k_param_arming = 100,

ArduPlane/Parameters.pde

@@ -1110,6 +1110,12 @@ const AP_Param::Info var_info[] PROGMEM = {
     GOBJECTN(gcs[2],  gcs2,       "SR2_",     GCS_MAVLINK),
 #endif
 
+#if MAVLINK_COMM_NUM_BUFFERS > 3
+    // @Group: SR3_
+    // @Path: GCS_Mavlink.pde
+    GOBJECTN(gcs[3],  gcs3,       "SR3_",     GCS_MAVLINK),
+#endif
+
     // @Group: INS_
     // @Path: ../libraries/AP_InertialSensor/AP_InertialSensor.cpp
     GOBJECT(ins,                    "INS_", AP_InertialSensor),

ArduPlane/system.pde

@@ -135,6 +135,11 @@ static void init_ardupilot()
     gcs[2].setup_uart(serial_manager, AP_SerialManager::SerialProtocol_MAVLink, 1);
 #endif
 
+#if MAVLINK_COMM_NUM_BUFFERS > 3
+    // setup serial port for fourth telemetry port (not used by default)
+    gcs[3].setup_uart(serial_manager, AP_SerialManager::SerialProtocol_MAVLink, 2);
+#endif
+
     // setup frsky
 #if FRSKY_TELEM_ENABLED == ENABLED
     frsky_telemetry.init(serial_manager);