build: Allow for updating an existing draft release

When creating a release draft and uploading assets, provision for the case
that a draft release already exists. In that case, instead of creating a
release, the existing release (changelog) is edited and the assets are
deleted before the new built ones are uploaded.

This commit also introduces the `${gh.repo}` property in build.xml,
so that the Github project could be chosen dynamically. Not really needed,
to be honest, but, yeah, whatever.

Author: flaix