units: Add new system-update-pre.target

units: Add new system-update-pre.target

systemd offline-updates allows dropping multiple system update units
to be added to system-update.target.wants.

As documented in systemd.offline-updates(7) only 1 of these units
should actually be active (based on the /system-update symlink) and
when that unit is done it should reboot the system.

In some cases it is desirable to run a unit whenever booting in
offline-updates mode indepedent of which update unit is going to
handle the update. One example of this is integration with bootloader
code which checks if the previous boot was succesful.

Since the active unit will reboot the system when it is done, there
is no guarantee that adding such a unit to system-update.target.wants
will get it executed always.

This commit adds a system-update-pre.target which can be used for
units which should always run when booting in offline-updates mode.