Allow rebuilding of a single element
Background
I have been altering the flatpak image plugin in bst-external and testing the change against freedesktop-sdk. If caching is enabled then attempting to rebuild against the change just causes objects to be pulled form the cache, and not being tested, however if I disable caching I have to wait for an ~6 hour build before seeing results. It would be nice to be able to rebuild only a specified element and use the cache for the rest.
Acceptance Criteria
A single element can be rebuilt without rebuilding dependencies.
Depends on #752 and #773 (closed)
Edited by Javier Jardón