stx-utilities: 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

- remove duplicated collect-engtools.inc

Issue-ID: INF-29
Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Change-Id: I64245da497621c59a47b3e80e92357d858e9903c