While attempting to fix the easy to mix up DIGESTS names in feb59db9f I
stumbled across yet another way that the DIGESTS names were a bit
unpredictable: previously a .bz2 got stuck into the file name when
upload_images automatically compressed some file types. The new code
missed this and never added the .bz2. Correct this now, both for
image_to_vm which has a pile of glue to keep the legacy names in place
for now and build_image which I never intended to change.
Make it possible for other scripts to share the same value for our
release repository and equally easy to override with a custom value.
Also allow setting the root from the command line in addition to the
environment. Usually --upload_root is better to use than --upload_path.
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.
