* 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.
The key scanning drivers are now automatically enabled when a DT node
with the matching "compatible" property is present and enabled, so they
no longer need to be manually set for each board.
