Add MarlinSettings::validate()

Marlin/src/gcode/eeprom/M500-M504.cpp

@@ -55,3 +55,15 @@ void GcodeSuite::M502() {
   }
 
 #endif // !DISABLE_M503
+
+#if ENABLED(EEPROM_SETTINGS)
+  /**
+   * M504: Validate EEPROM Contents
+   */
+  void GcodeSuite::M504() {
+    if (settings.validate()) {
+      SERIAL_ECHO_START();
+      SERIAL_ECHOLNPGM("EEPROM OK");
+    }
+  }
+#endif

Marlin/src/gcode/gcode.cpp

@@ -610,6 +610,9 @@ void GcodeSuite::process_parsed_command() {
       #if DISABLED(DISABLE_M503)
         case 503: M503(); break;  // M503: print settings currently in memory
       #endif
+      #if ENABLED(EEPROM_SETTINGS)
+        case 504: M504(); break;  // M504: Validate EEPROM contents
+      #endif
 
       #if ENABLED(ABORT_ON_ENDSTOP_HIT_FEATURE_ENABLED)
         case 540: M540(); break;  // M540: Set abort on endstop hit for SD printing

Marlin/src/gcode/gcode.h

@@ -681,6 +681,9 @@ private:
   #if DISABLED(DISABLE_M503)
     static void M503();
   #endif
+  #if ENABLED(EEPROM_SETTINGS)
+    static void M504();
+  #endif
 
   #if ENABLED(ABORT_ON_ENDSTOP_HIT_FEATURE_ENABLED)
     static void M540();