Let cgpt open devices in read-only mode when possible.

BUG=chromium-os:12430
TEST=manual

Running "make; make runtests" in src/platform/vboot_refererence will test
this change. Tests for use on a Chromebook are described in the bug report,
but will require a USB or SD card that has a physical write-protect switch.

Change-Id: I16a67bad3b59bec0981f4064f51fb1a29da65a90
Reviewed-on: https://gerrit.chromium.org/gerrit/21474
Tested-by: Bill Richardson <wfrichar@chromium.org>
Commit-Ready: Bill Richardson <wfrichar@chromium.org>
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Che-Liang Chiou <clchiou@chromium.org>
10 files changed