systemd: change dependency relationships

systemd: change dependency relationships

When building out a systemd based image, the udev-hwdb postinstall
script always fails at rootfs time. This is because that the 'udevadm'
command used in udev-hwdb postinstall script is now in the udev
package and udev recommends udev-hwdb.

Instead of letting udev recommends udev-hwdb, we let systemd do it
and make udev-hwdb rdepend on udev.

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