tunneldigger-broker: add broker for tunneldigger

tunneldigger-broker: add broker for tunneldigger

In mesh communities, tunneldigger is widely used to create L2TPv3 tunnels
and mesh via them. Since the broker is typically installed on other
distributions, the openwrt broker package has not received any
maintenance in recent years [0]. I  take now care of the further maintaince
of this package. Furthermore, I consulted with the maintainers to ensure
that they were comfortable with the change [1].

This PR is just a refactoring of the already existing opkg package from
wlanslovenija. It fixes config parsing and in general the config, adapts
to the new python syntax and fixes dependency handling.

- [0] https://github.com/wlanslovenija/firmware-packages-opkg/tree/master/net/tunneldigger-broker
- [1] https://github.com/wlanslovenija/firmware-packages-opkg/issues/24

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