package/radvd: disable by default in systemd preset-all

We don't provide a configuration file, so disable radvd by default.

Update the help message with instructions on how to enable radvd at
build time with systemd.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>