6relayd: make route preference and prefix on-link flag configurable

git-svn-id: svn://svn.openwrt.org/openwrt@36922 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: 6relayd: backport r36922

git-svn-id: svn://svn.openwrt.org/openwrt@36923 3c298f89-4303-0410-b956-a3cf2f4a3e73