Hello,
I followed the official documentation [1] to build U-Boot for my Rock Pi 4, instead of evb-rk3399_defconfig I used the config file for my board (rock-pi-4-rk3399_defconfig).
After I flashed the single boot image to an SD card and put it in I got stuck with the error message: "distro_bootcmd" not defined.
Since I have no idea of U-Boot it took me some time to find out that all I needed was to set a different default value for bootcmd. Now I use CONFIG_BOOTCOMMAND="bootflow scan" and it boots my OS as expected.
The official documentation looks straightforward for simple users like me, so I thought it might not be intended that the default value for the bootcmd entry is (still) distro_bootcmd.
Regards, Stefan.
[1] https://u-boot.readthedocs.io/en/latest/board/rockchip/rockchip.html