batman-adv: extend the ap_isolation mechanism

Change the AP isolation mechanism to not only "isolate" WIFI
clients but also all those marked with the more generic
"isolation flag" (BATADV_TT_CLIENT_ISOLA).

The result is that when AP isolation is on any unicast
packet originated by an "isolated" client and directed to
another "isolated" client is dropped at the source node.

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