Help porting an image to work with libre boards

I’ve read through some similar posts asking about this, but was still a bit fuzzy. I have tried the flash tool and portability tool to no avail. Can someone explain the basics of porting an OS and what is different from a raspberry pi to the libre boards (bootloader, kernel, etc.). I must admit I don’t know much about the process or how they interact with the hardware. I’d like to learn how to do it, so in the future I can port other images to work on this hardware. Any help is much appreciated!

We follow standards in every way possible and try to make our boards as close to standard PCs as possible. Images made for Raspberry Pi do not so we can only support very specific pathways. Anything outside of that pathway, we do not have the bandwidth for.