Hello,
As far I know, u-boot does not manage NFS mount point but Linux does. Please check your Linux has built-in NFS driver.
For sure, u-boot can support TFTP and ethernet to download a file (ie your kernel image) into memory, but you need to ensure that u-boot is built with these options. I don't have ethernet chip on my nanopi neo air, so I disabled eth completely on my u-boot build.
Regards.