I had the same problem. I was able to find a version of the rockchip factory tool that actually runs from the Rockchip site:
https://androidmtk.com/download-rockchip-factory-tool
I downloaded version 1.35 since that looked like Onyx was using. I used the OemImage.img file from the Onyx factory tool for the image that went in the demo field.
I still can't get the firmware image to install, I keep getting a Wait for MSC fail, but the factory tool from Rockchip runs otherwise.