cros_bundle_firmware: Tidy up exynos USB download
cros_bundle_firmware: Tidy up exynos USB download

Tighten up this code to reduce delays and release the power button on
servo as soon as possible.

BUG=chrome-os-partner:9633
TEST=manual:
Run 'cros_bundle_firmware -b daisy -w usb' 30 times and see that U-Boot
boots every time.

Change-Id: Ie58de9c011888c39c766d2cd8059ba41eb1c4387
Reviewed-on: https://gerrit.chromium.org/gerrit/23407
Commit-Ready: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Tested-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

marineam/coreos-dev-util,coreos/dev-util,mgireesh05/dev-util,mgireesh05/dev-util,coreos/dev-util,coreos/dev-util,mgireesh05/dev-util,marineam/coreos-dev-util