babeld: add add_filter function

babeld: add add_filter function

You can define filter functions in babeld by:

   in if eth0 metric 128

This commit adds the ubus equivalent to dynamically add filter on
runtime:

  ubus call babeld add_filter '{"ifname":"eth0", "type":"input",
                                "metric":128}'

Signed-off-by: Nick Hainke <vincent@systemli.org>