Add the initial support for PC-BSD to use multiple PKGNG repos

This will do the following:

It will allow us to maintain a single "full" package repo, for every XX.0-RELEASE
for the lifecycle of the entire 10.X, 11.X series, etc.

For specific XX.1 or XX.2 releases, or -STABLE, we will only
need to have our builders create a small set of packages specific
to that kernel / world. I.E drivers / kmod files, etc.

The main packages can still be pulled from the .0-RELEASE repo, since
they should be ABI compat