systemd: fix systemd-udev-hwdb-update service

systemd: fix systemd-udev-hwdb-update service

The new version of systemd has implemented the following feature.
Opointer.de/blog/projects/stateless.html

As a result, the systemd-udev-hwdb-update.service would always run
at first boot. This will cause failure if the target device doesn't
have enough storage space. Besides, as we run `udevadm hwdb --update' as
a postinst of udev-hwdb at rootfs time, there's no need to run this
again at system start-up.

The purpose of this patch is as follows.
If `udev hwdb --update' fails at rootfs time, systemd-udev-hwdb-update.service
is executed at first boot; otherwise, the service is not executed.

This patch achieves the above goal by setting CondistonNeedsUpdate to
"/etc/udev" in the service file, and creating /etc/udev/.updated file if the
postinst succeeds.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>