The MacBook 1,1 is the early 2006 model from Apple that shipped with an Intel Core Duo processor, 512MB of system memory, and sub-100GB hard drives. The enablement happened with this Git commit and basically rides off the 2,1 code. Given that the hardware specs aren't too different from the MacBook 2,1 models, it was fairly straightforward to support the older Apple laptops. There's also many other Core Duo / Core 2 era laptops and motherboards supported by Coreboot.
Those wanting to find motherboards they can use with Coreboot can see the Coreboot Wiki but sadly there's still little adoption of this former "LinuxBIOS" project by motherboard vendors and other IHVs outside the Chromebook world. There's also the rather ridiculous FSF-approved laptop that ships with Libreboot, a modified downstream of Coreboot.