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".
battery.c now uses the zmk,battery chosen node to select a battery
sensor. Using the node labeled "BATTERY" is maintained for backwards
compatibility but is now deprecated. Custom boards should switch to
using the chosen node.
# Conflicts:
# app/boards/arm/bluemicro840/bluemicro840_v1.dts
# app/boards/arm/nice60/nice60.dts
# app/boards/arm/nrfmicro/nrfmicro_13.dts
# Conflicts:
# app/boards/arm/bluemicro840/bluemicro840_v1.dts
