This variable works like CLEAN_DEPS, but applies to the distclean target.

Change-Id: Ia30e8932b9acd6529298728dd5d0e038b0208d66
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>