Plane: fixed home reset alt bias

this fixes a bug in the set of home alt while disarmed. The set had a
circular dependency which could cause a large bias to build up between
home alt and gps alt due to small fluctuations in barometer data

Thanks to Nick Allen for spotting this

ArduPlane/commands.cpp

@@ -122,7 +122,13 @@ void Plane::update_home()
     }
     if (ahrs.home_is_set() && !ahrs.home_is_locked()) {
         Location loc;
-        if(ahrs.get_position(loc)) {
+        if(ahrs.get_position(loc) && gps.status() >= AP_GPS::GPS_OK_FIX_3D) {
+            // we take the altitude directly from the GPS as we are
+            // about to reset the baro calibration. We can't use AHRS
+            // altitude or we can end up perpetuating a bias in
+            // altitude, as AHRS alt depends on home alt, which means
+            // we would have a circular dependency
+            loc.alt = gps.location().alt;
             if (!AP::ahrs().set_home(loc)) {
                 // silently fail
             }