systemd-conf: simplify creation of machine-specific configuration

systemd-conf: simplify creation of machine-specific configuration

The configuration files that systemd installs are just skeletons
detailing the available options and their default values.  The
recommended means of changing the configuration is to provide snippets
in configuration directories.  For example, journald.conf settings are
best set in /usr/lib/system.d/journald.conf.d/ and can be overridden by
the user by providing overriding snippets in
/etc/systemd/journald.conf.d/.

The systemd-conf package is just providing machine-specific overrides
for some systemd defaults.

This patch restores the installation of config files by systemd and
reduces systemd-conf to just providing the config snippets in
/usr/lib/systemd/*.conf.d.  This simpilfies the systemd-conf recipe
considerably since it now just sets up a couple of text files and
doesn't even need access to the systemd source anymore.

License-Update: configuration snippets licensing is independent of
systemd licensing

(From OE-Core rev: 3150253898babce70333376d22090b56b4a70bfb)

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