Nemo and NITDroid require a different kernel to be loaded in order to run. Bootmenu can not load alternative kernels. if you want to use Nemo without installing u-boot, you can flash the kernel from your p.c., and then use bootmenu to select the correct operating system for the kernel you loaded This is inconvenient.. The best way is to use u-boot.

Why do you want to avoid u-boot? It's the best available tool for what you're trying to do.