* rpi4: Enable arm_boost=1 to unlock 1.8Ghz CPU
The official Raspberry Pi OS enables a "boosted" 1.8GHz
mode since their Debian bullseye based release [source]. This
commit brings this feature to HA OS.
Support optional board specific default RPi firmware configuration file
(config.txt). Also rename from boot-env.txt to config.txt since this
file is not read by the U-Boot boot loader but the Raspberry Pi specific
boot firmware.