Serval multi key routing compatibility

This pull requests updates the commotiond serval plugin to work with the multi-key routing version of the serval-dna library. See https://github.com/opentechinstitute/serval-dna/pull/3 and https://github.com/opentechinstitute/commotion-feed/pull/51.

To test, build for OpenWRT and test all serval-related functionality:

1) serval-client commands (run `serval-client help` for list of available commands)
2) olsrd-mdp
3) service announcement