Plane: take advantage of rangefinder support in relative_ground_altitude()

ArduPlane/Plane.h

@@ -826,7 +826,7 @@ private:
     int32_t get_RTL_altitude();
     float relative_altitude(void);
     int32_t relative_altitude_abs_cm(void);
-    float relative_ground_altitude(void);
+    float relative_ground_altitude(bool use_rangefinder_if_available);
     void set_target_altitude_current(void);
     void set_target_altitude_current_adjusted(void);
     void set_target_altitude_location(const Location &loc);

ArduPlane/altitude.cpp

@@ -137,8 +137,12 @@ int32_t Plane::relative_altitude_abs_cm(void)
   return relative altitude in meters (relative to terrain, if available,
   or home otherwise)
  */
-float Plane::relative_ground_altitude(void)
+float Plane::relative_ground_altitude(bool use_rangefinder_if_available)
 {
+    if (use_rangefinder_if_available && rangefinder_state.in_range) {
+        return rangefinder_state.height_estimate;
+    }
+
 #if AP_TERRAIN_AVAILABLE
     float altitude;
     if (terrain.status() == AP_Terrain::TerrainStatusOK && terrain.height_above_terrain(altitude, true)) {