disable NFS support by default

While NFS is widely used in data centres, and private
networks it's quite a nuanced usecase for device firmware.
A lot of devices already disable it.

Various network protocols should really be opt in, not opt
out, because they add extra size and are potential attack
vectors from a security PoV. In the NFS case it doesn't
really make sense for a lot of devices like tables, SBCs etc.
It's also something we don't really want for SystemReady-IR
due to security concerns.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>