Update to rock1408

This PR updates the master/next/stable system in order to:
 - remove next (all packages in 'next' have been moved to 'stable')
 - make 'stable' track the latest release branch (in this case, rock1408)

without requiring any change in the autobuild code neither in the way most people work

It depends on the https://github.com/rock-core/package_set/pull/12 being applied