Create update related information per channel
We are currently creating update related information for all channels even if we just bump the version in a particular channel. Instead we should try to create only the update information (incremental .mar files; XML file updates...) needed for the particular channel: if we release a new stable version there should be no need to have an alpha build lying around for creating incremental stable .mar files. Nor should there be a need for updating alpha related XML files in this case.