Aktualizr reports incorrect OSTree hash if it doesn't find the currently-booting version in its targets

[tkfu]

When aktualizr assembles the device manifest, it calls OstreeManager::getCurrent() to assemble the information it needs. That function returns an Uptane::Target, so the current implementation is to try to locate a target matching the currently-running commit hash that aktualizr knows about, return that if we do find one, and return the Uptane::Target::Unknown() target if we don't find anything, which is just a target named "unknown" which reports the hash of an empty string as its hash.

This is incorrect behavior according to the Uptane spec: section 5.4.2.1.2 specifies that the ECU version report MUST contain "The filename, length, and hashes of its currently installed image".