generic: batman-adv: disable multicast optimizations for now

A bug in batman-adv can lead to a large amount of management traffic being
exchanged between nodes when the multicast optimizations are enabled,
effectively making the mesh unusable. It's safer to disable the feature
for now, until we have a real fix.

targets/generic

@@ -7,6 +7,8 @@ config 'CONFIG_BUSYBOX_CONFIG_SHA256SUM=y'
 config 'CONFIG_BUSYBOX_CONFIG_SHA512SUM=y'
 config '# CONFIG_BUSYBOX_CONFIG_FEATURE_PREFER_IPV4_ADDRESS is not set'
 
+config '# CONFIG_KMOD_BATMAN_ADV_MCAST is not set'
+
 config 'CONFIG_PACKAGE_ATH_DEBUG=y'
 
 config 'CONFIG_TARGET_MULTI_PROFILE=y'