Keyboard: Bpiphany Frosty Flake Refactor, Configurator support and readme cleanup (#4648)

* Frosty Flake: layout macro refactor

- frosty_flake.h updated to use #pragma once
- KEYMAP renamed to LAYOUT
- KEYMAP_TKL renamed to LAYOUT_tkl

* Frosty Flake: keymap refactor

- config.h files updated to #pragma once
- deleted deprecated build script rules from rules.mk
- keymap.c files:
  - #include QMK_KEYBOARD_H
  - updated layout macro name
  - white space changes for readability

* Frosty Flake: Configurator support

* Frosty Flake: readme cleanup

- Fixed Hardware Availability link (old link was 404)
- tiny grammar fix (colon missing on compile-and-flash instructions)

keyboards/bpiphany/frosty_flake/keymaps/default/config.h

@@ -1,8 +1,3 @@
-#ifndef CONFIG_USER_H
-#define CONFIG_USER_H
-
-#include "../../config.h"
+#pragma once
 
 // place overrides here
-
-#endif