The iso versions typically distributed by distros will only work on systems with a BIOS/EFI. Since the RockPro64 doesn't have a BIOS, the closest thing you can get to a bootloader is using Uboot. Different SoC (rockchip, allwinner, etc) have defined different locations for the boot sector/block. This causes the unfortunate situation where each SBC vendor needs its own specific rootfs images.
Here is a very good video that explains this whole process and what progress will be made going forward
https://fosdem.org/2019/schedule/event/hw_uboot/
Here is a very good video that explains this whole process and what progress will be made going forward
https://fosdem.org/2019/schedule/event/hw_uboot/
