ipv6-support: don't overwrite existing setting across reflashs

git-svn-id: svn://svn.openwrt.org/openwrt@35302 3c298f89-4303-0410-b956-a3cf2f4a3e73
ipv6: remove empty directory


git-svn-id: svn://svn.openwrt.org/openwrt@35303 3c298f89-4303-0410-b956-a3cf2f4a3e73