Booting the Thinkpad X220 now shows the coreboot logo splash screen instead of Lenovo's. The logo is a white rabbit running to the right on top of a coreboot wordmark on black background.
Testing the new coreboot firmware. You can see the Thinkpad has a keyboard loosely attached for testing and the screen shows the SeaBIOS boot prompt. The Raspberry Pi 4B is sitting disconnected on the side.
A Raspberry Pi 4B next to an open Thinkpad X220. It is connected directly to the BIOS chip with 8 jumper wires to allow flashing the firmware.
An 8-pin SOIC clip directly attached to the Thinkpad X220 BIOS chip. It allows reading out the firmware with a Raspberry Pi.
I removed the proprietary BIOS from my new Thinkpad X220 and flashed #coreboot with SeaBIOS. I also neutralized the Intel Management Engine. Bonus: I had spare space on the firmware chip, so I also added Tetris and Doom which I can play now even without any operating system.