Staging: batman-adv: add routing debug log accessible via debugfs

All routing debug messages are saved in a ring buffer that can be
read via the debugfs file "log".
Note that CONFIG_BATMAN_ADV_DEBUG must be activated to have the
debug logs compiled in.

Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>