Apologies, the links on how to change the Armbian boot kernel don't lead to any documentation. I am wading through the u-boot documentation tree now, but somebody may have an answer to a simple question.   I manage to make my device unbootable by using armbian-config and selecting a development kernel vmlinuz-4.20.0-rockchip64   I have mounted the disk on a ubuntu laptop and I take it u-boot is similar to grub2 in that its not just a case of changing the symlinks