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

Merged Adam requested to merge elagost/aports:brightnessctl-service into master

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.

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