Arria 10 SOC : How to build & generate Rootfs for Linux OS

we made an A10SOC customer board, We can build uboot & linux kernel now, but I don't know how to compile ROOTFS for SOC Linux OS. Is there any guidance of this topic , ?

Now we use Quartus Pro 18_1 and Linux Kernel is "Linux 4.9.78-ltsi-altera", Thanks in advance !!