cleanup: make nanopb an external git repo, referenced by git subtree

src/machinetalk/nanopb started as a copy of https://code.google.com/p/nanopb/ with local modifications. This was not a good idea with respect to tracking changes in nanopb. The solution is to fork nanopb, apply our local changes, delete the existing nanopb subtree, and bring in this fork with 'git subtree' in the same location.

That's what this cleanup merge does.

The nanopb code with minor mk modifications is now in https://github.com/mhaberler/nanopb (I'll transfer this to github.com/machinekit eventually). To track nanopb, our changes can be simply be rebased ontop of whatever happens in the nanopb source.

Related, the outdated copy of nanopb.proto in the machinetalk-protobuf repo has been removed; the build now refers to the current copy brought in with the subtree merge.

Update nanopb to 0.3.0 from 0.2.7.

Tested on all architectures.