ostbuild: Add ostree-revision in buildroots, bin-to-src

ostbuild: Add ostree-revision in buildroots, bin-to-src

First, for binary snapshots we need to include the exact revision of
the architecture buildroot.  To do that, introduce
"architecture-buildroots2".

Second, for bin-to-src, we'd like to allow "partial" builds.  So while
we do expand everything to source, *also* include the binary
ostree-revision.  This will allow building just one component, while
reusing the previously built binaries.