Add two release flavors for RISC-V. First, the traditional "big-iron" images,
capable of generating release tarballs and VM images. Installer images won't be
built yet, but can be trivially enabled in the future with the addition of
Second, a GENERICSD embedded image. I've opted for this instead of
board-specific SD card images as it allows users to just dd the u-boot they
want. The RISC-V hardware ecosystem is still young, so a configuration for e.g.
the HiFive Unleashed would likely see very few users.