Give the kernel a native NFS mount rpc capability for mounting NFS roots by

Give the kernel a native NFS mount rpc capability for mounting NFS roots by
splitting off the mount rpc code from the BOOTP code.  The loader is no
longer required to pass the nfs root mount file handle to the kernel.

Pure tftp-based loaders with no knowledge of NFS can now pass a NFS root
mount path to the kernel without having to pass a resolved NFS file handle.
This change allows kernels booted from tftp loaders to have an NFS root
without having to specify BOOTP (which sometimes doesn't work properly when
done from both the loader and from the kernel).