Plane: check that RTL_AUTOLAND is set if using DO_LAND_START

ArduPlane/AP_Arming.cpp

@@ -361,3 +361,16 @@ void AP_Arming_Plane::update_soft_armed()
     }
 }
 
+/*
+  extra plane mission checks
+ */
+bool AP_Arming_Plane::mission_checks(bool report)
+{
+    // base checks
+    bool ret = AP_Arming::mission_checks(report);
+    if (plane.mission.get_landing_sequence_start() > 0 && plane.g.rtl_autoland.get() == 0) {
+        ret = false;
+        check_failed(ARMING_CHECK_MISSION, report, "DO_LAND_START set and RTL_AUTOLAND disabled");
+    }
+    return ret;
+}

ArduPlane/AP_Arming.h

@@ -34,6 +34,7 @@ protected:
     bool ins_checks(bool report) override;
 
     bool quadplane_checks(bool display_failure);
+    bool mission_checks(bool report) override;
 
 private:
     void change_arm_state(void);