firmware: Prune down old region API

The region API was a way for firmware and kernel verification to get
at various blocks of caller-provided data.  In practice, we only used
it internally as a way to get at parts of the GBB.  Prune it down to
access only the bits of GBB we still need, from the buffer we already
know we have.

In the long run we should use the same vb2ex_read_resource() API that
vb2 firmware verification does, but that should be done in a follow-up
CL since it'll need to be coordinated with support in depthcharge.

No change in functionality.

BUG=chromium:611535
BRANCH=none
TEST=make -j runtests; build bob firmware and boot it

Change-Id: I5715cb8d88274164a1a73ed4a56bbd93af46f9bf
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/852798
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
25 files changed