BuildStream builds should bootstrap from Baserock binaries, not the Freedesktop SDK

See https://listmaster.pepperfish.net/pipermail/baserock-dev-baserock.org/2017-September/013811.html for a bit of context.

This merge request changes the BuildStream bootstrap elements to start from binaries that are stored in https://ostree.baserock.org/releases, which I originally built from the stage3-sysroot.bst element following the instructions in the README.

Support for more architectures will follow!

Please don't rebase this branch as the sysroot binaries contain a metadata file that refers back to a commit in this branch.

Here's what the metadata file looks like in the sysroot:

description: |
  This sysroot was produced by the Baserock project.
  See <http://www.baserock.org/> for more information.

project-repo: https://www.gitlab.com/baserock/definitions
project-commit: d1b8c40dd25d7419eed328eb5d11e6606cc7619b

build-tool: Buildstream
build-tool-version: bst, version 0.1.dev1422+g35d7051.d20171024