buildstream: Stop using deprecated BuildStream features

BuildStream is about to drop support for arch conditionals and pre-/post-commands.

This branch updates the Baserock definitions to use project options and conditionals to control the target architecture.

There is a corresponding defs2bst merge request that fixes the auto-converted .bst files.

The cross-bootstrap guide will need updating (and, ideally, re-testing) for these changes.

Fixes #14 (closed)