Merge pull request #3609 from xmcclure/checked-imageset

Merge pull request #3609 from xmcclure/checked-imageset

Fix outstanding failures in "metadata" checked build mode

Fix two problems with the checked build metadata mode which were causing spurious failures, one a bug introduced by my last commit, one of which has been present since the start:

- Check assemblies in `references[]` are non-NULL before looking up their images
- A "from" imageset only needs to reference every image in the "to" imageset, it does not need to actually contain every image.

Also added support for a checked build jenkins lane.