odhcp6c: Add missing config option "ifaceid"

git-svn-id: svn://svn.openwrt.org/openwrt@37033 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: odhcp6c: backport r37033

git-svn-id: svn://svn.openwrt.org/openwrt@37034 3c298f89-4303-0410-b956-a3cf2f4a3e73