Correctly handle reuse of screens in yaml file.

BUG=chromium-os:12161
TEST=manual

  cd src/platform/vboot_reference
  make
  make runbmptests

Change-Id: Ic7708474ce7009744c9f12154d1967bdae8e55d2

Review URL: http://codereview.chromium.org/6534025
3 files changed