Two new files in raspberry-pi/common/:
config-txt.nix lets you declare config.txt contents through
hardware.raspberry-pi.configtxt.settings (RFC 42 style — plain
values, null to remove, no enable/value wrappers).
config-txt-defaults.nix has the same defaults as Raspberry Pi OS:
vc4-kms-v3d, audio on, arm_boost, CM4 otg_mode, CM5 dwc2, etc.
All mkDefault so they are easy to override.
Not wired into board profiles yet — that comes with the RPi 5
sub-module work. Based on nvmd/nixos-raspberrypi.
a688dfcbb5