M420 support in MARLIN_DEV_MODE

Marlin/src/gcode/bedlevel/M420.cpp

  * With mesh-based leveling only:
  *
  *   C         Center mesh on the mean of the lowest and highest
+ *
+ * With MARLIN_DEV_MODE:
+ *   S2        Create a simple random mesh and enable
  */
 void GcodeSuite::M420() {
-  const bool seen_S = parser.seen('S');
-  bool to_enable = seen_S ? parser.value_bool() : planner.leveling_active;
+  const bool seen_S = parser.seen('S'),
+             to_enable = seen_S ? parser.value_bool() : planner.leveling_active;
+
+  #if ENABLED(MARLIN_DEV_MODE)
+    if (parser.intval('S') == 2) {
+      #if ENABLED(AUTO_BED_LEVELING_BILINEAR)
+        bilinear_start[X_AXIS] = MIN_PROBE_X;
+        bilinear_start[Y_AXIS] = MIN_PROBE_Y;
+        bilinear_grid_spacing[X_AXIS] = (MAX_PROBE_X - (MIN_PROBE_X)) / (GRID_MAX_POINTS_X - 1);
+        bilinear_grid_spacing[Y_AXIS] = (MAX_PROBE_Y - (MIN_PROBE_Y)) / (GRID_MAX_POINTS_Y - 1);
+      #endif
+      for (uint8_t x = 0; x < GRID_MAX_POINTS_X; x++)
+        for (uint8_t y = 0; y < GRID_MAX_POINTS_Y; y++)
+          Z_VALUES(x, y) = 0.001 * random(-200, 200);
+      SERIAL_ECHOPGM("Simulated " STRINGIFY(GRID_MAX_POINTS_X) "x" STRINGIFY(GRID_MAX_POINTS_X) " mesh ");
+      SERIAL_ECHOPAIR(" (", MIN_PROBE_X);
+      SERIAL_CHAR(','); SERIAL_ECHO(MIN_PROBE_Y);
+      SERIAL_ECHOPAIR(")-(", MAX_PROBE_X);
+      SERIAL_CHAR(','); SERIAL_ECHO(MAX_PROBE_Y);
+      SERIAL_ECHOLNPGM(")");
+    }
+  #endif
 
   // If disabling leveling do it right away
   // (Don't disable for just M420 or M420 V)