Add 'karaf-framework' feature to describe core Karaf bundles

The current trunk provides a 'karaf-framework' feature to describe the core karaf bundles. It would be nice to have a similar feature available in Karaf 2.x as well.

Currently, if any feature is installed that refers to a core bundle and then is uninstalled again, the features service thinks that the bundle is no longer used by any features and will try to uninstall the bundle as well. With this additional feature, we could add it to the list of boot features to avoid running into this scenario and have a nice XML description of the core Karaf bundles in one go.