VPP-43 Separating NSH from transport - setup for move to NSH_SFC repo

Keith Burns <alagalah@...>

Please seeĀ https://gerrit.fd.io/r/#/c/1015/

Dave Barach and Hongjun Ni have both reviewed and commented.

If this is merged I can then:
- add GRE support to existing GRE code base
- move NSH folder to NSH_SFC repo and make it a plugin (first phase cut/paste to get it out of VPP, next convert to plugin once in correct repo).