core: disable "update" units in the initramfs

Initially I wanted to add ConditionPathExists=!/etc/initrd-release in various
units (ldconfig.service, systemd-sysusers.service, systemd-hwdb-update.service,
systemd-journal-catalog-update, systemd-update-done.service), but I think it's
better to just disable the mechanism in the initrd altogether. Initrd images
are put together in a very particular way, and there is not need to do
post-update steps on them. If a unit from some other package winds up in the
initrd, we wouldn't want to invoke it either.

Also, any modifications are ephemeral, so any update would happen on every
use. And finally, initrd images are all about speed, and we shouldn't invoke
any unneeded services.

(cherry picked from commit 294eace0)