This repository contains the CoreOS branches of Linux. A new branch is created for each kernel version the distribution is rebased onto. The currently shipped version is available in the overlay repository.
This repository is not used by the build process at all; it merely exists for generating patchsets that live in coreos-overlay. To add a patch, check out the branch you want to add a patch to (probably v${VERSION}-coreos), apply the patch, then run git format-patch ${TAG} where tag is the upstream kernel tag (no "-coreos"). This generates a set of patches that can be used by ./revbump.sh script in the coreos-sources directory in the overlay.
git checkout v4.13.16-coreos
<Apply and commit your patch>
git format-patch v4.13.16
# check that the patches were generated correctly
ls *.patch