r26926 moved all target features to subtargets if present, which had the
side effect that broken targets are available for selection, just without
any available subtargets (since they are still marked as broken).

Fix this by explicitly letting the top level target also depend on broken.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@33892 3c298f89-4303-0410-b956-a3cf2f4a3e73