Rover: Reset home to AHRS location rather then snapshotting GPS

Also corrects rover locking home altitude to EKF origin altitude

APMrover2/commands.cpp

@@ -95,17 +95,12 @@ void Rover::restart_nav()
 void Rover::update_home()
 {
     if (home_is_set == HOME_SET_NOT_LOCKED) {
-        Location loc = gps.location();
-        Location origin;
-        // if an EKF origin is available then we leave home equal to
-        // the height of that origin. This ensures that our relative
-        // height calculations are using the same origin
-        if (ahrs.get_origin(origin)) {
-            loc.alt = origin.alt;
+        Location loc;
+        if (ahrs.get_position(loc)) {
+            ahrs.set_home(loc);
+            Log_Write_Home_And_Origin();
+            GCS_MAVLINK::send_home_all(gps.location());
         }
-        ahrs.set_home(loc);
-        Log_Write_Home_And_Origin();
-        GCS_MAVLINK::send_home_all(gps.location());
     }
     barometer.update_calibration();
 }