* Use extra comptible = "zmk,behavior-global" to add
behaviors to global bindings for event notification.
* Implement mod-tap, as a keymap binding and global one
to skip tap if other keycode pressed while held.
* 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.
