net: Export auto_load, use it in rarp

The rarp code includes another instance of the auto_load logic, so call
what is now net_auto_load() instead.

This also fixes an incorrect call to TftpStart() which was never seen
since apparently no boards enable rarp.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>