Example for HI3516EV200 device
bootargsnfs=mem=${osmem:-32M} console=ttyAMA0,115200 panic=20 root=/dev/nfs rootfstype=nfs ip=dhcp nfsroot=192.168.1.254:/media/nfs/hi3516ev200,v3,nolock rw ip=192.168.1.55:192.168.1.254:192.168.1.254:255.255.255.0::eth0
nfsboot=tftp 0x42000000 uImage;setenv setargs setenv bootargs ${bootargsnfs};run setargs;bootm 0x42000000
run nfsboot
How to boot OpenIPC with NFS and TFTP without replacing stock firmware (russian post): https://habr.com/ru/companies/ruvds/articles/774482/