batman-adv: Make sysfs support optional

The sysfs files will be marked as deprecated in the near future. They are
already replaced by the batadv generic netlink family. Add an Kconfig
option to disable the sysfs support for users who want to test their tools
or want to safe some space. This setting should currently still be enabled
by default to keep backward compatible with legacy tools.

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