I dislike the elinux upstream guide because it's more confusing and complicated (U-Boot is not necessary). I suggest to use a default raspbian stretch lite, add the upstream DT blob and replace the kernel7.img with your build zImage. In order to use the upstream DT blob you will need to modify the config.txt like here
. More helpful information: https://github.com/anholt/linux/wiki/Ra ... nvironment
I suggest to use the latest upstream kernel 4.15, but there are still missing features:
CPU frequency driver
BCM2835 aux irqchip driver
In case of issues you can contact linux-rpi-kernel (moderated list).