batman-adv: Prepare framework for mesh genl config

The batman-adv configuration interface was implemented solely using sysfs.
This approach was condemned by non-batadv developers as "huge mistake".
Instead a netlink/genl based implementation was suggested.

The main objects for this configuration is the mesh/soft-interface object.
Its actual object in memory already contains most of the available
configuration settings. The genl interface reflects this by allowing to
get/set it using the mesh specific commands.

The BATADV_CMD_GET_MESH_INFO (or short version BATADV_CMD_GET_MESH) is
reused as get command because it already provides the content of other
information from the mesh/soft-interface which are not yet configuration
specific.

The set command BATADV_CMD_SET_MESH will also notify interested userspace
listeners of the "config" mcast group using the BATADV_CMD_SET_MESH command
message type that settings might have been changed and what the current
values are.

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