* Start of work of focusing on out-of-tree user keymaps.
Add ZMK_CONFIG support.
* General external configuration supported via ZMK_CONFIG,
which should point to a configuration directory.
* Priority order of overlay files to select.
* ${SHIELD}_${BOARD}.overlay
* ${SHIELD}.overlay
* ${BOARD}.overlay
* default.overlay
* Equivalent search order for .conf files for initial Kconfig
values.
* Properly allow includes for keymaps to allow defining
custom keycodes, and then adding them to the
keymap.overlay file.
* Fix keymap.c loading after build refactor.
* Fix for deactivating a layer working properly.
* Fix default keymap to put transparent in proper spots to
make raise/lower keycodes work as expected.
* Add custom raise/lower keycode handling to default
keymap.
* 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.
Pete Johanson