UEFI platforms may provide a watchdog timer, which will reboot the
machine if an operating system takes more than five minutes to load.
This can cause long-lived iPXE downloads (or interactive shell
sessions) to unexpectedly reboot.

Fix by resetting the watchdog timer every ten seconds while the iPXE
main processing loop continues to run.

Reported-by: Bradley B Williams <bradleybwilliams@swbell.net>
Reported-by: John Clark <john.r.clark.3@gmail.com>
Reported-by: wdriever@gmail.com
Reported-by: Charlie Beima <cbeima@indiana.edu>
Signed-off-by: Michael Brown <mcb30@ipxe.org>