batman-adv: provide orig_node routing API

Some operations executed on an orig_node depends on the
current routing algorithm being used. To easily make this
mechanism routing algorithm agnostic add a orig_node
specific API that each algorithm can populate with its own
routines.

Such routines are then invoked by the code when needed,
without knowing which routing algorithm is currently in use

With this patch 3 API functions are added:
- orig_free (to free routing depending internal structs)
- orig_add_if (to change the inner state of an orig_node
  when a new hard interface is added)
- orig_del_if (to change the inner state of an orig_node
  when an hard interface is removed)

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