StarFive VisionFive and HiFive Unmatched SD Images

Two new images are available:

• nixos-sd-image-visionfive.zst: Minimal SD image for the StarFive VisionFive board.
• nixos-sd-image-unmatched.zst: Minimal SD image for the HiFive Unmatched. It requires U-Boot to be installed to the SPI flash beforehand (see README).

Note that the images were still built on an older nixpkgs version from January. A rebased tree is currently being built. Other images (e.g., QEMU) are available in past releases.

Thanks to StarFive/RVspace for sponsoring the VisionFive board ❤️