buildscript: update batman-adv (fix gw selection)

Signed-off-by: Tim Niemeyer <tim.niemeyer@mastersword.de>
Reviewed-by: Christian Dresel <fff@chrisi01.de>
Reviewed-by: Tobias Klaus <tk+ff@meskal.net>

buildscript

@@ -30,7 +30,7 @@ OPENWRT_PKGS="gpioctl-sysfs libugpio fastd haserl"
 ## See http://www.open-mesh.org/projects/batman-adv/wiki/Compatversion
 BATMAN_ADV=(batman_adv
             https://github.com/freifunk-gluon/packages.git
-            b4f04f51d53b151a45f0618eef6d89d32f52dae7)
+            bdb56bba02168a7fdd7c8bbf380ae59a4febac7c)
 BATMAN_ADV_PKGS="kmod-batman-adv-legacy micrond simple-tc"
 
 #official openwrt routing packages