net: lwip: tftp: bind to TFTP port only when in server mode

net: lwip: tftp: bind to TFTP port only when in server mode

The TFTP app should not bind to the TFTP server port when configured as
a client. Instead, the local port should be chosen from the dynamic
range (49152 ~ 65535) so that if the application is stopped and started
again, the remote server will not consider the new packets as part of
the same context (which would cause an error since a new RRQ would be
unexpected).

Submitted upstream: https://savannah.nongnu.org/patch/?10480

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>