It seems that the /mnt/tftp was copied from the defaultenv-2. The
defaultenv-2 got a update to make it more robust by enabling all
interfaces. During this sync I added a entry to support automount for
/mnt/nfs mountpoint.

Cc: Stefan Christ <s.christ@phytec.de>
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>