pxe: use a separate poll thread instead of using an idle thread hook

pxe: use a separate poll thread instead of using an idle thread hook

The idle thread can never sleep, so it's not really safe to do
anything inside it.  Instead, run a separate poll thread at low
priority; we can also do that to poll the serial console if needed.

Overall, the "classic" Syslinux idle handling really should go away
and be replaced by the idle thread.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>