Rootfs
Debian
Yu can download the prebuit image directly.
or you could use the rootfs-build-script from Rockchip to build rootfs yourselves.
Yocto
If you want to use Yocto to build your rootfs see yocto-user-guide for detail.
Buildroot
If you want to use Buildroot to build your rootfs see buildroot-user-guide for detail.