systemd: create preset files instead of installing in image

systemd: create preset files instead of installing in image

At first boot, systemd will create the /etc/systemd/system directory
from service preset files.  As such, for a normal, writable /etc
(writable rootfs), there is no need to set up this directory at image
creation time.

This patch changes the systemd machinery to create preset files and to
rely on systemd to do the service enablement.

This breaks the read-only-rootfs case; there's a fix for this in a
follow-up patch.

(From OE-Core rev: 154abbc3296eded11d2bbe3e102470b6986d42cd)

Signed-off-by: Jonas Bonn <jonas@norrbonn.se>
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>