# basics
git clone https://github.com/ARM-software/arm-trusted-firmware.git && cd arm-trusted-firmware
make -j4 PLAT=rk3328 bl31
cd ..
git clone https://gitlab.denx.de/u-boot/u-boot.git
cd u-boot
echo CONFIG_SD_BOOT=y >> configs/rock64-rk3328_defconfig
echo CONFIG_EXT4_WRITE=y >> configs/rock64-rk3328_defconfig
cp ../arm-trusted-firmware/build/rk3328/release/bl31/bl31.elf bl31.elf
make -j4 rock64-rk3328_defconfig

Results:
idbloader.img
u-boot.itb

Flash:
dd if=idbloader.img of=/dev/xxx seek=64
dd if=u-boot.itb of=/dev/xxx seek=16384