updater: unpack FMAP only when required.

For platforms using incomplete or buggy EC FMAP during development time, we need
to use FMAP only when really being required.

BUG=chrome-os-partner:11934
TEST=emerge-daisy chromeos-firmware-daisy

Change-Id: I9b3eb8511492e54f221633cf00e064b1780c77bc
Reviewed-on: https://gerrit.chromium.org/gerrit/28648
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Reviewed-by: Bernie Thompson <bhthompson@chromium.org>
Reviewed-by: Yung-Chieh Lo <yjlou%chromium.org@gtempaccount.com>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Commit-Ready: Hung-Te Lin <hungte@chromium.org>