That's true. They don't officially support the Raspberry Pi because it needs proprietary drivers and thus breaks the "chain of trust" needed for flashing Libreboot.
It's a bit over-the-top paronoid to expect firmware modifying software in GPU drivers, but I understand their point.
To call the binary blobs on the Pi "GPU drivers" is an understatement. The VC4 is running the full ThreadX operating system. The Pi is closer to a VC4 board with a secondary ARM coprocessor tacked on than it is to an ARM board.
All the same, is your adversary really going to use this to attack your machine? They'd have to predict a number of things quite in advance, as well as sneak an attack into the blob.
> The Raspberry Pi boots from its GPU and only non-free software is currently available for the GPU, even starting the machine requires a large (2MB) blob of non-free, unsupportable software
The VC4 GPU actually is the primary chip; it boots the ThreadX operating system on the VC4 first, then ThreadX loads the user-configured system onto the ARM core. Whether you run headless or not, ThreadX is running on the VC4.
Could you direct me to a tutorial on that? I contacted the Libreboot devs before and they told me they wouldn't give any info out about using a different device
