Add method to retrieve version in firmware preamble

This is to enable retrieve firmware version, firmware datakey version,
kernel subkey version

BUG=chrome-os-partner:12442
TEST=Check we can retrieve firmware version, firmware datakey version,
  kernel subkey version

Change-Id: Ia116fafd9279eb7d367f20ce85bade5594ed37a3
Reviewed-on: https://gerrit.chromium.org/gerrit/29721
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Commit-Ready: Chun-Ting Chang <ctchang@chromium.org>
Tested-by: Chun-Ting Chang <ctchang@chromium.org>
2 files changed