Hello folks. I'm wanting to learn a bit about computer hardware and firmware design, the ultimate goal will be a fully open-source hardware computer (I don't expect that any time soon). I'm familiar with PCB layout and design already as well as MCU and general programming.

Does anyone have suggestions for Off-the-Shelf CPUs that are supported well-enough by Linux and have useful documentation and datasheets available? I'm not looking for high performance, running a GUI, or anything like that. I'm literally just interested in practicing the board layout and figuring out how to extend core/libreboot to support it (out implement my own firmware) and get a terminal session.