batman-adv: make the backbone gw check VLAN specific

The backbone gw check has to be VLAN specific so that code
using it can specify VID where the check has to be done.

In the TT code, the check has been moved into the
tt_global_add() function so that it can be performed on a
per-entry basis instead of ignoring all the TT data received
from another backbone node. Only TT global entries belonging
to the VLAN where the backbone node is connected to are
skipped.
All the other spots where the TT code was checking whether a
node is a backbone have been removed.

Moreover, batadv_bla_is_backbone_gw_orig() now returns bool
since it used to return only 1 or 0.

Cc: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>