stx-metal: inherit systemd to enable services

stx-metal: inherit systemd to enable services

- inherit systemd bbclass and set correct variables to
  enable services.

- remove sysvinit from DISTRO_FEATURES_BACKFILL_CONSIDERED
  to avoid init scripts being removed.

- remove the useless pkg_postinst functions

- move lmon.service to mtce-lmon

Issue-ID: INF-28
Issue-ID: INF-29
Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Change-Id: Id17ce7b5536686ab180c0714c9355143adcc3d8d