saft: Move cmp_boot_vector() to ChromeOSInterface class

Make it reusable from other places.

BUG=chromium-os:26953,chromium-os:32871
TEST=run FAFT firmware_TryFwB/control.normal on Link passed.

Change-Id: I53456b583389e1672feb844f98ef6334d0258d24
Reviewed-on: https://gerrit.chromium.org/gerrit/28575
Reviewed-by: Vic Yang <victoryang@chromium.org>
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
2 files changed