For multi-file uploads we should explicitly declare what the name of the
.DIGESTS file should be instead of using the first file name. Relying on
the ordering was subtle and easy to break.
Sync up bootstrap_sdk with other tools by using the common upload
functions. As part of this refactor release_util a bit to provide a
truly generic upload function.
This will be used to upload the latest images built from master, we
don't need every build so we just want to upload to a 'master'
directory, not one named for the current version.
