vboot: save GBB header in workbuf during firmware verification

Since GBB header will be needed for subsequent GBB reads later on
(in kernel verification stage), and since GBB header is
relatively small (128 bytes), save the full GBB header onto
workbuf during firmware verification stage, and store an offset
pointer to it in vb2_shared_data.  vb2_gbb_header object may be
accessed via the vb2_get_gbb function.

Additionally, update functions in firmware/lib/region-init.c to
read GBB data from flash, rather than using cparams passed in by
depthcharge, which is slated for deprecation.

BUG=b:124141368, chromium:954774
TEST=make clean && make runtests
BRANCH=none

Change-Id: I6e6218231299ce3a5b383663bc3480b20f929840
Signed-off-by: Joel Kitching <kitching@google.com>
Cq-Depend: chromium:1585500
Reviewed-on: https://chromium-review.googlesource.com/1627430
Commit-Ready: Joel Kitching <kitching@chromium.org>
Tested-by: Joel Kitching <kitching@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Joel Kitching <kitching@chromium.org>
29 files changed