rootfs-postcommands: update systemd_create_users

Process all systemd-sysusers configuration files in order to create
users/groups at build time. systemd-sysusers would try to create them at
run-time, but for read-only rootfs that's not possible and results in
warnings from different services:

systemd-udevd[166]: /lib/udev/rules.d/50-udev-default.rules:39 Unknown group 'render', ignoring
systemd-udevd[166]: /lib/udev/rules.d/50-udev-default.rules:40 Unknown group 'render', ignoring
systemd-udevd[166]: /lib/udev/rules.d/50-udev-default.rules:42 Unknown group 'sgx', ignoring

Signed-off-by: Vyacheslav Yurkov <uvv.mail@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit f86ffdb1)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>