Generated content.xml is wrong in some cases

The plugin uses tycho 2.7.5 which generates the wrong content.xml. See eclipse-tycho/tycho#2160.
So either

• update tycho to a more recent version or
• provide an option to skip the FeaturesAndBundlesPublisher and CategoryPublisher invokation so useres can run the tycho-p2-extras-plugin publish-features-and-bundles goal themself.