br2684ctl: resolve a boot time race condition with nas0 bringup by using explicit notification when init is done

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48321 3c298f89-4303-0410-b956-a3cf2f4a3e73