vboot_ref: merge struct firmware_image;

BUG=b:207808292
BRANCH=none
TEST=none

Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Change-Id: I49bd29dcfe12ad9845213d07bc6bee71980a8cd1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3461092
Tested-by: Edward O'Callaghan <quasisec@chromium.org>
Auto-Submit: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-by: Nikolai Artemiev <nartemiev@google.com>
Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Edward O'Callaghan <quasisec@chromium.org>