Sync before/after setting mesh_bed_leveling_flag/homing_flag

mesh_bed_leveling_flag and homing_flag should be set only when the
planner is empty as a consequence of #3100, since any operation executed
*while* the mbl/homing flag is set is considered part of the compound
instruction that generated it (G80, G28, G30, G76).

Failure in doing so can result in instructions just prior being lost,
since they're incorrectly discarded (assumed to be redone).

G1  ; lost ..
G80 ; .. if PP happens here

Firmware/Marlin_main.cpp

@@ -2660,16 +2660,15 @@ static void gcode_G28(bool home_x_axis, long home_x_value, bool home_y_axis, lon
 static void gcode_G28(bool home_x_axis, long home_x_value, bool home_y_axis, long home_y_value, bool home_z_axis, long home_z_value, bool without_mbl)
 #endif //TMC2130
 {
+	// Flag for the display update routine and to disable the print cancelation during homing.
 	st_synchronize();
+	homing_flag = true;
 
 #if 0
 	SERIAL_ECHOPGM("G28, initial ");  print_world_coordinates();
 	SERIAL_ECHOPGM("G28, initial ");  print_physical_coordinates();
 #endif
 
-	// Flag for the display update routine and to disable the print cancelation during homing.
-	homing_flag = true;
-
 	// Which axes should be homed?
 	bool home_x = home_x_axis;
 	bool home_y = home_y_axis;
@@ -2912,24 +2911,21 @@ static void gcode_G28(bool home_x_axis, long home_x_value, bool home_y_axis, lon
 
 #if (defined(MESH_BED_LEVELING) && !defined(MK1BP))
 	if (home_x_axis || home_y_axis || without_mbl || home_z_axis)
-		{
+    {
       if (! home_z && mbl_was_active) {
         // Re-enable the mesh bed leveling if only the X and Y axes were re-homed.
         mbl.active = true;
         // and re-adjust the current logical Z axis with the bed leveling offset applicable at the current XY position.
         current_position[Z_AXIS] -= mbl.get_z(st_get_position_mm(X_AXIS), st_get_position_mm(Y_AXIS));
       }
-		}
-	else
-		{
-			st_synchronize();
-			homing_flag = false;
-	  }
+    }
 #endif
 
 	  if (farm_mode) { prusa_statistics(20); };
 
+      st_synchronize();
 	  homing_flag = false;
+
 #if 0
       SERIAL_ECHOPGM("G28, final ");  print_world_coordinates();
       SERIAL_ECHOPGM("G28, final ");  print_physical_coordinates();
@@ -2950,6 +2946,10 @@ static void gcode_G28(bool home_x_axis, bool home_y_axis, bool home_z_axis)
 // G80 - Automatic mesh bed leveling
 static void gcode_G80()
 {
+    st_synchronize();
+    if (waiting_inside_plan_buffer_line_print_aborted)
+        return;
+
     mesh_bed_leveling_flag = true;
 #ifndef PINDA_THERMISTOR
     static bool run = false; // thermistor-less PINDA temperature compensation is running
@@ -3350,9 +3350,11 @@ static void gcode_G80()
     lcd_setstatuspgm(_T(WELCOME_MSG));
     custom_message_type = custom_message_type_old;
     custom_message_state = custom_message_state_old;
-    mesh_bed_leveling_flag = false;
     mesh_bed_run_from_menu = false;
     lcd_update(2);
+
+    st_synchronize();
+    mesh_bed_leveling_flag = false;
 }
 
 
@@ -5145,8 +5147,9 @@ if(eSoundMode!=e_SOUND_MODE_SILENT)
     */
     case 30: 
         {
-            homing_flag = true;
             st_synchronize();
+            homing_flag = true;
+
             // TODO: make sure the bed_level_rotation_matrix is identity or the planner will get set incorectly
             int l_feedmultiply = setup_for_endstop_move();
 
@@ -5249,7 +5252,9 @@ if(eSoundMode!=e_SOUND_MODE_SILENT)
             }
         }
 
+        st_synchronize();
         homing_flag = true; // keep homing on to avoid babystepping while the LCD is enabled
+
         lcd_update_enable(true);
         KEEPALIVE_STATE(NOT_BUSY); //no need to print busy messages as we print current temperatures periodicaly
         SERIAL_ECHOLNPGM("PINDA probe calibration start");