systemd: start device units directly
systemd: start device units directly

A few systemd.conf attendees pointed out that device units could be
started directly. We'll use that approach instead of the transient
units.

dgonyeo/ignition,higebu/ignition,ajeddeloh/ignition,coreos/ignition,higebu/ignition,higebu/ignition,AlexNPavel/ignition,coreos/ignition,coreos/ignition,ajeddeloh/ignition,ajeddeloh/ignition,AlexNPavel/ignition,dgonyeo/ignition,dgonyeo/ignition,AlexNPavel/ignition