batman-adv: Add network_coding mesh genl configuration

The mesh interface can use (in an homogeneous mesh) network coding, a
mechanism that aims to increase the overall network throughput by fusing
multiple packets in one transmission.

The BATADV_CMD_SET_MESH/BATADV_CMD_GET_MESH commands allow to set/get the
configuration of this feature using the BATADV_ATTR_NETWORK_CODING_ENABLED
attribute. Setting the u8 to zero will disable this feature and setting it
to something else is enabling this feature.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>