fs/tftp: handle incoming packets in the separate tftp_recv() function
The separation of incoming packets handling makes it much easier
to run barebox tftp client on top of picotcp network stack in the future.
Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>