batman-adv: add bat_orig_print API function

Each routing protocol has its own metric and private
variables, therefore it is useful to introduce a new API
for originator information printing.

This API needs to be implemented by each protocol in order
to provide its specific originator table output.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>