odhcp6c: export passthru options as netifd data

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40825 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/network/ipv6/odhcp6c/files/dhcpv6.script

@@ -77,6 +77,7 @@ setup_interface () {
 
 	proto_add_data
 	[ -n "$CER" ] && json_add_string cer "$CER"
+	[ -n "$PASSTHRU" ] && json_add_string passthru "$PASSTHRU"
 	proto_close_data
 
 	proto_send_update "$INTERFACE"