nfsroot: Default mount option should ask for built-in NFS version

Change the nfsroot default mount option to ask for NFSv2 only *if* the
kernel was built with NFSv2 support.
If not, default to NFSv3 or as last choice to NFSv4, depending on actual
kernel config.

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>