Add BuildStream conversion script and do ongoing BuildStream convert + build as part of the CI

This involves merging the gnu-toolchain conversion branch that Tristan Van Berkom did in the buildstream-tests repo.

Part of #11 (closed).

There are various limitations to this; so far it can really just be used for building x86_64 systems. Deploying them requires extra changes, as does merging in the various existing non-x86_64 .morph files into the .bst files in a logical way (we can reduce a lot of duplication when we do this, but the actual work has to be done manually).