batman-adv: send announcement when backbone gw is registered

To avoid loops in the startup phase until the first announcement is
sent, send an announcement immediately as soon as a backbone gw is
added.

This may happen due to various reasons, e.g. a packet passes the rx
or tx path.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>