handwired/promethium Refactor and Configurator support (#4524)

* handwired/promethium: refactor

- config.h files
  - updated to use #pragma once
  - removed redundant config.h includes
- layout macro moved from config.h to promethium.h
- layout macro renamed from KEYMAP_CUSTOM to LAYOUT

* handwired/promethium: Configurator support

keyboards/handwired/promethium/keymaps/priyadi/config.h

@@ -1,7 +1,4 @@
-#ifndef CONFIG_USER_H
-#define CONFIG_USER_H
-
-#include "../../config.h"
+#pragma once
 
 #define PRIYADI_PROMETHIUM
 
@@ -23,5 +20,3 @@
 
 #define DOUBLESPACE_LAYER_ENABLE
 // #define TOLELOT_ENABLE
-
-#endif