vboot2: Add routine to verify kernel preamble

This also checks that the bootloader and vmlinuz headers, if present,
are within the signed part of the kernel blob; the vboot1 routines
didn't do that.  That wasn't harmful at firmware boot time because the
vboot1 routines would only load as much data as was signed, but in
vboot2 loading the kernel data is the responsibility of the caller so
we need to check.

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

Change-Id: I73eb4831e5d3d7a642b6cb85cb55857d87fcc0af
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/270797
6 files changed