You can make this default on your hardware independent of the OpenWrt image you installed it on. Then at every power cycle the router have a new MAC address. This would be option a) in this thread. When you remove the MAC address from your hardware, then at every boot it does not know what MAC address to set and it choose a random one. This also make sure that its impossible to forget to configure this option and leak your MAC address that is printed on the bottom of the device because you make it impossible to the Linux Kernel that is booting up to know what MAC address is printed on the label: