SCRIPTS: improve announce-release to support different tag and versions

SCRIPTS: improve announce-release to support different tag and versions

By having three variables it will be easier to preset the version and
the tag separately. One contains the announced version, another one the
associated tag and the last one the final commit ID (used as the ending
point before the release). This initially allows to check for the HEAD
matching the tag only when the version was not forced, hence re-announce
already tagged versions after some extra commits were added for example.