vboot2: Add 2.0 api layer to verify kernel partition

This allows the caller to load the kernel partition and then pass it
to vboot for verification, rather than having vboot assume the kernel
partitions are all on a block storage device.

Next up, APIs for the caller to parse partition information from a GPT
(yes, that's cgptlib, but we'll make it more easily callable by
depthcharge).

BUG=chromium:487699
BRANCH=none
TEST=make -j runtests

Change-Id: I388085c7023f4c76d416f37df0607019bea844ac
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/275646
Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>
7 files changed