community/brightnessctl: add service to save/restore backlight settings

This is a simple service file to save/restore backlight settings on shutdown/boot. I have been using a service like this for a while since I switched to Alpine, since it doesn't save and restore brightness settings automatically like some other systemd-based distros do.

I am open to suggestion on where to put the 'save' file. It has to be on persistent storage, though, since it must survive a reboot.