Add MANUAL_PROBE_START_Z for manual probing

Marlin/Configuration.h

  * or (with LCD_BED_LEVELING) the LCD controller.
  */
 //#define PROBE_MANUALLY
+//#define MANUAL_PROBE_START_Z 0.2
 
 /**
  * A Fix-Mounted Probe either doesn't deploy or needs manual deployment.

Marlin/src/config/default/Configuration.h

  * or (with LCD_BED_LEVELING) the LCD controller.
  */
 //#define PROBE_MANUALLY
+//#define MANUAL_PROBE_START_Z 0.2
 
 /**
  * A Fix-Mounted Probe either doesn't deploy or needs manual deployment.

Marlin/src/feature/bedlevel/bedlevel.cpp

 
   void _manual_goto_xy(const float &rx, const float &ry) {
 
-    #if MANUAL_PROBE_HEIGHT > 0
+    #ifdef MANUAL_PROBE_START_Z
+      #if MANUAL_PROBE_HEIGHT > 0
+        do_blocking_move_to(rx, ry, MANUAL_PROBE_HEIGHT);
+        do_blocking_move_to_z(MAX(0,MANUAL_PROBE_START_Z);
+      #else
+        do_blocking_move_to(rx, ry, MAX(0,MANUAL_PROBE_START_Z);
+      #endif
+    #elif MANUAL_PROBE_HEIGHT > 0
       const float prev_z = current_position[Z_AXIS];
       do_blocking_move_to(rx, ry, MANUAL_PROBE_HEIGHT);
       do_blocking_move_to_z(prev_z);