run-postinsts: Add systemd service file

run-postinsts: Add systemd service file

This patch mainly adds a systmd service file for run-postinsts,
which is started at first boot to run the post-install scripts.

Apart from this, this patch also modifies the installation location
of run-postinsts to ${sbindir}. This is because this script would be
used by both sysvinit and systemd based images. So it's more reasonable
to make it locate under ${sbindir}.

[YOCTO #5719]

(From OE-Core rev: 5933fbef26ffbc8140248ffb28957f36a813054b)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>