daisy: Deal with no-timeout explicity in wait_in_signal()
daisy: Deal with no-timeout explicity in wait_in_signal()

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 <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

alterapraxisptyltd/chromium-ec,md5555/ec,coreboot/chrome-ec,longsleep/ec,thehobn/ec,akappy7/ChromeOS_EC_LED_Diagnostics,mtk09422/chromiumos-platform-ec,mtk09422/chromiumos-platform-ec,coreboot/chrome-ec,thehobn/ec,eatbyte/chromium-ec,coreboot/chrome-ec,fourier49/BIZ_EC,longsleep/ec,fourier49/BZ_DEV_EC,gelraen/cros-ec,gelraen/cros-ec,mtk09422/chromiumos-platform-ec,gelraen/cros-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BIZ_EC,fourier49/BIZ_EC,fourier49/BZ_DEV_EC,coreboot/chrome-ec,longsleep/ec,md5555/ec,akappy7/ChromeOS_EC_LED_Diagnostics,eatbyte/chromium-ec,thehobn/ec,alterapraxisptyltd/chromium-ec,coreboot/chrome-ec,alterapraxisptyltd/chromium-ec,thehobn/ec,akappy7/ChromeOS_EC_LED_Diagnostics,gelraen/cros-ec,eatbyte/chromium-ec,coreboot/chrome-ec,fourier49/BIZ_EC,md5555/ec,longsleep/ec,eatbyte/chromium-ec,akappy7/ChromeOS_EC_LED_Diagnostics,mtk09422/chromiumos-platform-ec,alterapraxisptyltd/chromium-ec,md5555/ec,fourier49/BZ_DEV_EC,fourier49/BZ_DEV_EC