Buildroot User Guide

Build buildroot rootfs:
Enter into buildroot working directory

cd buildroot

Compile the rootfs,Generate the rootfs in buildroot/output/images directory. 
make rockchip_rk3399_defconfig
make

You can use following integrative cmds and scripts to build firmware:

cd buildroot && make rockchip_rk3399_defconfig && cd .. && ./build_all.sh
&& ./mkfirmware.sh

In order to get more details,you could refer to <RK3399 Linux Buildroot System Development Guide.pdf> in 'docs' directory.