fix(cros_sdk): Use SDK version from version.txt in the manifest repo

Second stage of moving version information from coreos-overlay to the
manifest.
clean(upload_prebuilts): Our upload code is in the scripts repo

Removing this removes the last reference to
coreos-overlay/coreos/binhost which I would also like to delete.
Merge pull request #7 from marineam/sdk-version

Sdk version