busybox: FEATURE_IPV6 depends on FREETZ_TARGET_IPV6_SUPPORT (refs #1786)

git-svn-id: file:///var/svn/freetz/trunk@9943 149334a1-2f27-0410-a3b9-fc62619ac1e6
busybox: set symbols to "no" if added dependencies are not met (refs #1786)

git-svn-id: file:///var/svn/freetz/trunk@9944 149334a1-2f27-0410-a3b9-fc62619ac1e6