* Previous version of multiple overrides of the default value of ZMK_USB
were problematic. Move to using board _defconfig files for the
defaults for those, along with proper `depends on` for ZMK_USB that
accounts for split and split roles.
* Add blackbill interconnect metadata.
* Add config/overlay for upstream blackpill boards to work with ZMK.
* Add pillbug board.
* Updated new shield docs.
Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>