batman-adv: Move from DEBUGFS to batctl

The debug filesystem will be deprecated and disabled by default
in newer versions of B.A.T.M.A.N.

This patch switches our code to the batctl, as recommended.

Since batctl can suppress headers, this actually makes our life
easier ...

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>