systemd-update-done.service, systemd-update-done — Mark /etc and /var fully updated
systemd-update-done.service
/usr/lib/systemd/systemd-update-done
systemd-update-done.service is a
    service that is invoked as part of the first boot after the vendor
    operating system resources in /usr have been
    updated. This is useful to implement offline updates of
    /usr which might requires updates to
    /etc or /var on the
    following boot.
systemd-update-done.service updates the
    file modification time (mtime) of the stamp files
    /etc/.updated and
    /var/.updated to the modification time of the
    /usr directory, unless the stamp files are
    already newer.
Services that shall run after offline upgrades of
    /usr should order themselves before
    systemd-update-done.service, and use the
    ConditionNeedsUpdate= (see
    systemd.unit(5))
    condition to make sure to run when /etc or
    /var are older than /usr
    according to the modification times of the files described above.
    This requires that updates to /usr are always
    followed by an update of the modification time of
    /usr, for example by invoking
    touch(1)
    on it.