In order to build the kernel, download this toolchain and extract it to your /opt directory.
Then, place your rootfs.cpio.gz image you got from the Buildroot3DS build process and place it in this directory.
After that, run './make_3ds.sh' and hold enter until you see compiler text.
Literally, I am not kidding. Hold that enter key down. It will still work with you just holding enter.
linux3ds-kernel is based off the "linux" repository from the "Linux for 3DS" team and the original linux repository.