I've built FriendlyWrt from the source that I recently retrieved from the repository.

When I try to boot it, it fails due to not being told where its root filesystem is.

I've got round this by using make menuconfig in u-boot to modify the configs/nanopi_r2_defconfig configuration to
include the boot option root=/dev/mmcblk0p8

(The partition number may vary).

But how was this meant to work?