Removed "label" properties which no longer have any function.
Labels are still used as layer names and as identifiers for sending
behaviors between sides of a split keyboard, so those have been left
alone for now.
Changed the label property on zmk,ext-power-generic to be optional and
removed it from existing uses. Renamed the nodes for all non-development
boards to "EXT_POWER" to preserve user settings.
rgb_underglow.c now finds the correct device by finding the first
instance of zmk,ext-power-generic instead of looking for a node named
"EXT_POWER".
* 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.
* Move away from DT_LABEL.
* Move to DEVICE_DT_GET for non-behavior device access.
* Move various drivers to `gpio_spec_dt` and `DT` related macros.
* Remove mcp23017 while at it, since better upstream driver is
available.
