there is upstream code but it's not well-integrated, and google's tree is largely optimised for their own payload called Deptchcharge. mrchromebox tweaks coreboot for use with edk2 so you can use normal operating systems, and the modifications that they make are extensive, e.g. mechanism in EDK2 to update EC firmware at boot time. mrchromebox is, for our purposes, a coreboot distro or indeed its own upstream. libreboot takes mrchromebox and makes several of its own modifications. "just use coreboot" is easier said than done :)