batman-adv: ABI: Mark sysfs files as deprecated

The sysfs files are replaced by the batadv generic netlink family. The old
sysfs configuration interface was frowned upon by other kernel developers.
But the files cannot be removed immediately because userspace tools might
still depend on it. Instead schedule for its removal in 2021.

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