Matthew Garrett

That is untrue. The issue your post doesn't discuss is that the ChromeOS security model relies on hardware write-protect to the flash chip containing the firmware as the core in it's security model, so you need to disable the hardware write-protect and then run the script that Duncan posted in the comments above (/usr/share/vboot/bin/make_dev_firmware.sh) to get the ChromeOS firmware on with whatever keys you specify as an argument to the script. The ChromeOS team's biggest problem here is not documenting this process better.