This function accepts a timeout value of -1, so add a comment to that
effect. Also make it deal with this explicitly since the current function
seems to return prematurely in that case.
BUG=chrome-os-partner:9424
TEST=very ad-hoc:
1. build and boot on daisy, flash U-Boot with USB using
'cros_bundle_firmware -w usb', inserting daisy
USB cable when it says 'Reseting board via servo...'
2. Press cold reset, then power on, see that it powers on
3. Then hold power-on for 8 seconds and see that it power off
4. XPSHOLD function not tested yet
Change-Id: I01bd81997836333ca33e61d48823e8ff41034d5f
Signed-off-by: Simon Glass <sjg@chromium.org>