weston-init: guard against systemd configs

Just as sysvinit scripts shouldn't be present in a distro using systemd,
systemd scripts shouldn't be present in a system not using systemd.

Signed-off-by: Randolph Sapp <rs@ti.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 00e3eb38)
Signed-off-by: Steve Sakoman <steve@sakoman.com>