Add method to read SHA-1 hash of firmware section in flashrom handler

This is to enable checking if firmware section has changed.

BUG=chrome-os-partner:11980
TEST=Check we can read SHA-1 hash.

Change-Id: I35ecf0a3faf43c70c9ec7dc2e6d5f8b9276adeb3
Reviewed-on: https://gerrit.chromium.org/gerrit/28790
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Vic Yang <victoryang@chromium.org>
Commit-Ready: Vic Yang <victoryang@chromium.org>