config: Allow buildex params from overlays

Previously, build/clean/process were all spec'ed to merge only the build and run sections from any overlays. I believe this was intended to allow future additions to the config to not cause issues (e.g. if a 'configure' command and section was ever added, it doesn't make sense to pull that into a build since configure has to have already happened).

However when 'buildex' was added as a back compat extension for the build section, it was not treated as part of the build section and erroneously filtered out. Let's fix that.

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