Zyxel EX5601 - Stock layout

Hello everyone,

On the firmware selector for Zyxel EX5601, there are 2 option, first is (OpenWrt U-Boot layout) and 2nd is (stock layout).

I am able to install Openwrt on EX5601 using the first option with no problem following the instructions given Here.

However, I would like to know how to install OpenWrt using the second option of (stock layout), I understood that (stock layout) is there if you want to follow the manufacture flash layout, but I cannot find any detailed instructions about how to install the second option of (stock layout).

Could someone here give the full instructions to install OpenWrt on Zyxel EX5601 using (stock layout)?

Thank you in advance.