babeld: add add_interface function

babeld: add add_interface function

An interface can be added dynmiacally to babeld by starting babeld with
the local management interface (-G) and saying:
  interface eth0

Add the ubus equivalent of this function:
  ubus call babeld add_interface '{"ifname":"eth0"}'

Signed-off-by: Nick Hainke <vincent@systemli.org>
(cherry picked from commit 385200443554ab87e0205f5551f7a0c1883da776)