convert bbswitch job to systemd unit

dramatically speeds up my boot time because it was the last
service (for me) that depended on udev-settle.service

udev-settle isn't needed for modern system initialization but some
oldschool services (mdadm/lvm/cryptsetup) depend on it so they can
just enumerate devices instead of having to react to changes
dynamically. In NixOS these things are usually already taken care of
during stage 1 (early ramdisk) if you use them.
power management: provide a post-resume target and unify
suspend/hibernate/hybrid-sleep handling