build_library: add torcx upload/download roots
build_library: add torcx upload/download roots

Torcx is special in that it wishes to be uploaded under a prefixed
directory (torcx), typically wishes to be downloaded from there, but
ultimately wants to be downloaded from a location without that prefix.

In fact, I expect during a normal release process, it will be uploaded
with that prefix to the build bucket, copied without that prefix to the
final bucket (during pre-release), and then finally downloaded without
the prefix.

I think this set of variables ends up being the cleanest way to
represent this complexity.

coreos/scripts,coreos/scripts,glevand/coreos--scripts,glevand/coreos--scripts