Good news, I have found a workaround for this. I can use the host tools mkimage to generate the boot.scr : https://github.com/flatmax/buildroot.rockchip/blob/master/board/RK3566.cm3/post-image.sh#L32
$HOST_DIR/bin/mkimage -C none -A arm -T script -a 0x00c00000 -e 0x00c00000 -n ‘flatmax load script’ -d $BR2_EXTERNAL_RK3308_PATH/board/RK3566.cm3/boot.cmd $BINARIES_DIR/boot.scr
Booting now works.
It appears that there is a bug in the radxa uboot repo w.r.t. the mkimage script.