batman-adv: add support for the isolation_mark option

With the introduction of the Extended Isolation component[1]
a new option is required by batman-adv: the isolation_mark.
This patch adds support for it in the configuration file

[1] http://www.open-mesh.org/projects/batman-adv/wiki/Extended-isolation

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>