package/avahi: use upstream-provided systemd files

When using the buildroot-provided avahi-daemon.service file, bootup never
finishes, because multi-user.target is waiting for avahi-daemon to exit,
which is caused by "Type=oneshot" (in avahi-daemon.service).

Upstream's systemd files get already installed to /lib/systemd.
They're not an exact copy of S50avahi-daemon, but work flawlessly,
so use these units:

* avahi-daemon.service, auto-enabled (ln -fs in AVAHI_INSTALL_INIT_SYSTEMD)
* avahi-daemon.socket,
  not auto-enabled, but a dependency of avahi-daemon.service
* avahi-dnsconfd.service, auto-enabled

[Thomas: use simpler absolute paths for the symbolic links instead of
relative paths. Suggested by Maxime Hadjinlian.]

Signed-off-by: André Erdmann <dywi@mailerd.de>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>