Plane: only apply DO_LAND_START arming check on quadplanes

on fixed wing aircraft you can use DO_LAND_START with DO_GO_AROUND. On
QuadPlane there is no reason to have DO_LAND_START without
RTL_AUTOLAND

ArduPlane/AP_Arming.cpp

@@ -368,9 +368,15 @@ 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) {
+#if HAL_QUADPLANE_ENABLED
+    if (plane.quadplane.available() &&
+        plane.mission.get_landing_sequence_start() > 0 &&
+        plane.g.rtl_autoland.get() == 0) {
         ret = false;
+        // a quadplane with DO_LAND_START and no RTL_AUTOLAND set is almost certainly a config error
+        // note that non-quadplanes can use DO_LAND_START with DO_GO_AROUND
         check_failed(ARMING_CHECK_MISSION, report, "DO_LAND_START set and RTL_AUTOLAND disabled");
     }
+#endif
     return ret;
 }