batman-adv: Trigger genl notification on sysfs config change

The generic netlink code is expected to trigger notification messages when
configuration might have been changed. But the configuration of batman-adv
is most of the time still done using sysfs. So the sysfs interface should
also trigger the corresponding netlink messages via the "config" multicast
group.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>