config: Allow artifacts to be omitted from final copy stage

Create an alternative dictionary format for artifact fields,
where the new keys are:
- "path": location of the artifact
- "export": true (default) if the artifact should be copied, else false
- "rename": (optional) rename the artifact while copying

Flag unexported dependencies in the run section instead of
skipping them during string substitution.

Signed-off-by: Yuliang Wang <yuliang.wang@arm.com>