package/systemd: add setting for systemd default.target

package/systemd: add setting for systemd default.target

This adds the option to set systemd's default.target in
the System Configuration subheading if systemd is
specified as the init system.

The argument for default.target is specified pre-build
as opposed to overriding the hardcoded "multi-user.target"
symlink with post-build scripts or a rootfs overlay

Signed-off-by: Sen Hastings <sen@phobosdpl.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>