net: Add tftp command which uses the new tftp code ...

Traditionally tftp is a command of its own. Since we can
mount tftp as a filesystem we have duplicated the tftp code, the old
code is used by the tftp command, the new by the filesystem support.
This adds a tftp command based on the filesystem support to preserve
backward compatibility. This allows us to remove the old tftp support
in the next step.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>