This reverts commit c7220ca802.
The removal was done as a reaction to the messages from systemd:
initrd-root-fs.target: Requested dependency OnFailure=emergency.target ignored (target units cannot fail).
initrd.target: Requested dependency OnFailure=emergency.target ignored (target units cannot fail).
initrd-root-device.target: Requested dependency OnFailure=emergency.target ignored (target units cannot fail).
initrd-fs.target: Requested dependency OnFailure=emergency.target ignored (target units cannot fail).
local-fs.target: Requested dependency OnFailure=emergency.target ignored (target units cannot fail).
...
But it seems that the messages themselves are wrong, and the units were OK.
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).
Yu Watanabe