See c80a9a33d0, target units can't fail.
I guess we need to figure out some replacement functionality, but at least
let's avoid the warning from systemd for now.
Add a synchronization point so that custom initramfs units can run
after the root device becomes available, before it is fsck'd and
mounted.
This is useful for custom initramfs units that may modify the
root disk partition table, where the root device is not known in
advance (it's dynamically selected by the generators).
