Thanks to Vladimir Serbinenko, as of today there's now mainline support for the Lenovo X220. The Lenovo ThinkPad X220 is an Intel Sandy Bridge era laptop/ultra-portable. While Sandy Bridge isn't exactly the newest hardware out there, it's better than some of the recently enabled Coreboot devices and certainly much better than the Free Software Foundation prompted re-brand of the Lenovo ThinkPad X60 with its Libreboot downstream of Coreboot.
The specific ThinkPad X220 model being advertised is 4299Y48 and the support landed with this Git commit. Another commit also added native Sandy Bridge support.
Lenovo X220 owners wishing to learn more about what works and what doesn't with Coreboot at this time in replacing the proprietary BIOS, see the Coreboot Wiki.