[2.0.x] Add O (optional) flag to G28/G29 (#10913)

Marlin/src/gcode/bedlevel/abl/G29.cpp

  *
  * Enhanced G29 Auto Bed Leveling Probe Routine
  *
+ *  O  Auto-level only if needed
+ *
  *  D  Dry-Run mode. Just evaluate the bed Topology - Don't apply
  *     or alter the bed level data. Useful to check the topology
  *     after a first run of G29.
   // Don't allow auto-leveling without homing first
   if (axis_unhomed_error()) return;
 
+  if (!no_action && planner.leveling_active && parser.boolval('O')) { // Auto-level only if needed
+    #if ENABLED(DEBUG_LEVELING_FEATURE)
+      if (DEBUGGING(LEVELING)) {
+        SERIAL_ECHOLNPGM("> Auto-level not needed, skip");
+        SERIAL_ECHOLNPGM("<<< G29");
+      }
+    #endif
+    return;
+  }
   // Define local vars 'static' for manual probing, 'auto' otherwise
   #if ENABLED(PROBE_MANUALLY)
     #define ABL_VAR static

Marlin/src/gcode/calibrate/G28.cpp

  *  None  Home to all axes with no parameters.
  *        With QUICK_HOME enabled XY will home together, then Z.
  *
+ *  O   Home only if position is unknown
+ *
  *  Rn  Raise by n mm/inches before homing
  *
  * Cartesian/SCARA parameters
     }
   #endif
 
+  if ((axis_known_position[X_AXIS] && axis_known_position[Y_AXIS] && axis_known_position[Z_AXIS]) && parser.boolval('O')) { // home only if needed
+    #if ENABLED(DEBUG_LEVELING_FEATURE)
+      if (DEBUGGING(LEVELING)) {
+        SERIAL_ECHOLNPGM("> homing not needed, skip");
+        SERIAL_ECHOLNPGM("<<< G28");
+      }
+    #endif
+    return;
+  }
+  
   // Wait for planner moves to finish!
   planner.synchronize();