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.

boards/shields/petejohanson_handwire/keymaps/default/keymap.c

@@ -0,0 +1,7 @@
+
+#include <zmk/keys.h>
+
+bool zmk_handle_key_user(struct zmk_key_event *key_event)
+{
+	return true;
+};