TS-7990 NFS boot

U-Boot includes support for NFS client which can be used to load the kernel, device tree binary, and root filesystem across the network. Our default environment contains the nfsboot command which can be updated to boot NFS on a custom network:

# Set this to your NFS server IP
env set nfsroot 192.168.0.36:/mnt/storage/imx6/
env save
# Boot to NFS once
run nfsboot;

# To make the NFS boot the persistent default
env set bootcmd run nfsboot;
env save