These are the prebuilt output for the HDMI2USB misoc firmware.
Most of these builds are created via Travis builds.
The structure is;
archive/$(git describe)/$TARGET/$BOARD/
Example;
archive/v0.0.0-152-ge2b8b92/base/atlys/
archive/v0.0.0-152-ge2b8b92/base/opsis/
archive/v0.0.0-152-ge2b8b92/hdmi2usb/atlys/
archive/v0.0.0-152-ge2b8b92/hdmi2ethernet/opsis/
These directories are symlinked as stable/testing/unstable from other directories.