Quite a number of the arm boards have open UEFI ports. Tianocore edk-platforms https://github.com/tianocore/edk2-platforms/tree/master/Platform https://github.

Quite a number of the arm boards have open UEFI ports. Tianocore edk-platforms https://github.com/tianocore/edk2-platforms/tree/master/Platform https://github.com/tianocore/edk2-platforms/tree/master/Plat... has a number that are maintained together. Others can be found in various places on github and/or a vendors site. Of note, the rpi3/4 are both supported. The hardware isn't particularly open, but they have been reverse engineered to the point where even the videocore firmware has a open source project ongoing to replace it. There are also various tutorials for getting inexpensive jtag probes working with openocd on the rpi3/4 so its possible to have a fully opensource firmware development stack. Edit: There are a couple intel ports as well, if that is one's preferred platform.