vboot2: Add host lib function to create a vb2-style keyblock

Also add vb2_common_desc() helper function to return the description
for an object starting with a common struct header.

And use the new host lib function to create the keyblock for verifying
the firmware lib.

Add tests for everything new.

BUG=chromium:423882
BRANCH=none
TEST=VBOOT2=1 make runtests

Change-Id: I1fadb3e249e771a692cc69b23620c6ddd46a48ac
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/231721
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
11 files changed