I found Lenovo X220, T420s and T520 have coreboot support, but T420 is not in the support list. I used the autoport tool in coreboot, and found some differences(e.g. GPIO) between T420 and the supported models. I think it should be easy to port it.

I've just disassembled a Lenovo ThinkPad T420, and found a Winbond 25Q64CV flash after I moved the mainboard out. However, I couldn't read it with my flash programmer. I don't know if the flash can be read if I try another programmer or SOIC-8 clip.