systemd: Don't install resolv.conf symlink when resolved is disabled

systemd: Don't install resolv.conf symlink when resolved is disabled

There's no point in letting systemd install it, if systemd doesn't
provide DNS. Let other services like resolvconf or connman do that,
because only they know the location they expect.

This also fixes a problem during do_rootfs with apt and dpkg's
update-alternatives:

| update-alternatives: error: alternative path /etc/resolv-conf.systemd doesn't exist
| dpkg: error processing package systemd:armhf (--configure):
|  subprocess installed post-installation script returned error exit status 2

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>