batman-adv: Allow to disable debugfs support

The files provided by batman-adv via debugfs are currently converted to
netlink. Tools which are not yet converted to use the netlink interface may
still rely on the old debugfs files. But systems which already upgraded
their tools can save some space by disabling this feature. The default
configuration of batman-adv on amd64 can reduce the size of the module by
around 11% when this feature is disabled.

    $ size net/batman-adv/batman-adv.ko*
       text    data     bss     dec     hex filename
     150507   10395    4160  165062   284c6 net/batman-adv/batman-adv.ko.y
     137106    7099    2112  146317   23b8d net/batman-adv/batman-adv.ko.n

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>