implement release channels

[f321x]

Implements release channels so the user can select either a stable or beta release channel in the SettingsDialog for update notifications. Adapts the release_www.sh and make_download scripts to handle alpha and beta versions which are detected by an 'a' or 'b' in the version name. The version fields are added into the extradata field of the version file as version_alpha and version_beta and the extradata field is being signed too in the extradata_hash_signature field. The update checker is supposed to return the newest version of those contained in the version file which is allowed according to the user configured option.