avahi: Don't advertise example services by default

The example service files are placed into /etc/avahi/services when we
run `make install` for avahi. This results in ssh and sftp-ssh services
being announced by default even if no ssh server is installed in an
image.

These example files should be moved away to another location such as
/usr/share/doc/avahi (taking inspiration from Arch Linux).

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>