Uptane should not default to using two kinds of hashes in fileinfo (image info)

[awwad]

Currently, Uptane operates with tuf.conf.REPOSITORY_HASH_ALGORITHMS set to ['sha256', 'sha512'], TUF's default, which results in fileinfo (Targets role(s) metadata about images) being generated using both types of hashes. In order to save space in messages to Secondaries in-vehicle, we should use one type of hash, especially for partial verification secondaries. Currently, there is a related bug in TUF that makes this a problem, so we're going to wait until it's fixed in TUF.