The images we release are the result of building coreboot using the configuration in our git repository at https://github.com/merge/skulls
git clone https://github.com/merge/skulls
The coreboot source code is available via git:
git clone https://review.coreboot.org/coreboot.git
In case there are external patches applied to build coreboot, they should be up for review at review.coreboot.org and included in the Skulls git repository's common/download_coreboot.sh