batman-adv: throw uevent in userspace on gateway add/change/del event

In case of new default gw, changing the default gw or deleting the default gw a
uevent is triggered with type=gw, action=add/change/del and
data={GW_ORIG_ADDRESS} (if any).

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>