config: Use ${btvar:XXX} in more config sections

Allow btvars to be additionally resolved in build..{sourcedir, builddir, repo, toolchain} sections. People have use cases for sourcedir and repo (both remote and revision). I'm adding builddir and toolchain speculatively.

Note that the implementation actually allows most of the macros to be resolved in these sections, but I'm only documenting the above because other macros might not make sense in these sections.

Signed-off-by: Ryan Roberts ryan.roberts@arm.com