batman-adv: add sysfs framework for VLAN

Each VLAN can now have its own set of attributes which are
exported through a new subfolder in the sysfs tree.
Each VLAN created on top of a soft_iface will have its own
subfolder.

The subfolder is named "vlan%VID" and it is created inside
the "mesh" sysfs folder belonging to batman-adv.

Attributes corresponding to the untagged LAN are stored in
the root sysfs folder as before.

This patch also creates all the needed macros and data
structures to easily handle new VLAN spacific attributes.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>