test: add NFS download test

Add a NFS download test, based on TFTP test.
Tested on i.MX6 SabreLite board.

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>

Cc: Tom Rini <trini@konsulko.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>