AC_Fence: add get_boundary_update_ms

this allows callers to detect changes in the underlying polygon fence

libraries/AC_Fence/AC_Fence.cpp

@@ -573,6 +573,7 @@ bool AC_Fence::load_polygon_from_eeprom()
         _boundary[index] = ekf_origin.get_distance_NE(temp_loc) * 100.0f;
     }
     _boundary_num_points = _total;
+    _boundary_update_ms = AP_HAL::millis();
 
     // update validity of polygon
     _boundary_valid = _poly_loader.boundary_valid(_boundary_num_points, _boundary);

libraries/AC_Fence/AC_Fence.h

@@ -112,6 +112,9 @@ public:
     /// handler for polygon fence messages with GCS
     void handle_msg(GCS_MAVLINK &link, mavlink_message_t* msg);
 
+    /// return system time of last update to the boundary (allows external detection of boundary changes)
+    uint32_t get_boundary_update_ms() const { return _boundary_update_ms; }
+
     static const struct AP_Param::GroupInfo var_info[];
 
     // methods for mavlink SYS_STATUS message (send_sys_status)
@@ -187,6 +190,7 @@ private:
     uint8_t         _boundary_num_points = 0;       // number of points in the boundary array (should equal _total parameter after load has completed)
     bool            _boundary_create_attempted = false; // true if we have attempted to create the boundary array
     bool            _boundary_valid = false;        // true if boundary forms a closed polygon
+    uint32_t        _boundary_update_ms;            // system time of last update to the boundary
 };
 
 namespace AP {