https://github.com/rockchip-toybrick rk3399 source code download address
Just set the environment variable before compiling: export CROSS_COMPILE=” aarch64-linux-gnu-”
http://t.rock-chips.com/wiki.php?mod=view&id=42 u-boot compilation
http://t.rock-chips.com/wiki.php?mod=view&id=26 kernel compilation