net: lwip: nfs: Prefer nfsserverip over serverip when set

net: lwip: nfs: Prefer nfsserverip over serverip when set

Prefer use of a 'nfsserverip' env var before falling back to 'serverip'
when using the nfs command. Similar to how the 'tftpserverip' env var
is preferred over 'serverip' by the tftp command.

This also updates the error message to closer match the error message
used by the lwIP tftp command when a server ip is not set.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Jerome Forissier <jerome.forissier@arm.com>