batman-adv: Add the backbone gateway list to debugfs

This is especially useful if there are no claims yet, but we still want
to know which gateways are using bridge loop avoidance in the network.

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