* Move to ZephyrBuildConfig package based on Zephyr best practices.
* Support multiple shields properly, e.g. -DSHIELD="foo bar", which
allows using existing upstream shields like `ssd1306_128x64` along
with another ZMK shield.
* Properly *add* the "ZMK config" .conf file to the conf file list,
so that existing board .conf files in `app/boards/<board>.conf` are
properly honored still. Needed for overrides to usptream boards when
combined with a local `<shield>.conf` file.