Refactor build/config/keymaps.

* Move all headers to `include/zmk` directory.
* Update includes to reference them properly.
* Add `keymap.c` file from keymap, if found.
* Toplevel CONFIG_ZMK_BLE aggregate config setting.

src/hid.c

@@ -1,4 +1,4 @@
-#include "hid.h"
+#include <zmk/hid.h>
 
 static struct zmk_hid_report report = {
     .modifiers = 0,
@@ -53,4 +53,4 @@ int zmk_hid_release_key(zmk_key code)
 struct zmk_hid_report *zmk_hid_get_report()
 {
     return &report;
-}
+}