initscripts: update postinst

in container image, it don't have init system,
install package initscripts will report error:
systemctl: command not found

fix by use same way as systemd.bbclass to decide if systemctl
mask will run

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>