* Needed so when building with `kyria_left` shield, then config
dir containing `kyria.conf` and `kyria.overlay` will also be
applied as configs/overlays.
* 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.
