I am doing linux from scratch using 64-bit openSUSE. I would also like to do this with the openSUSE OS running from USB. I spent an entire day trying to get this OS to run from USB yesterday. I almost did, because it definitely booted grub, but it was checking the microSD port to boot from. I followed instructions that I believe are outdated, because I followed them exactly at each step of the way. I definitely changed a grubfile that referenced the sd to reference USB. The file was called dracula.raspberry, or something, I'm not looking right at it right now.
The instructions were also updated at the end to include the fact that the capability is now built into some tumbleweed versions with a link to those versions. I'm pretty sure that it was one of these that I tried that booted grub.
I also booted a regular image from usb with a blank microSD card inserted, this did the same as the other.
I tested the usb to see that it is able to boot by using Raspbian, and it booted fine.
I really want to boot this from USB, or at least boot from microSD, and the microSD direct the system to the USB.
Is source code available to the public for openSUSE? Also, is source code available to the public for Raspbian? I would like to read these to see exactly what is going on without just relying on the solution outline that I found for how the raspberry pi boots.