vboot: Add multiboot kernel type

Add a kernel type for signing multiboot kernel images.

BUG=b:38040849
BRANCH=none
TEST=properly sign a multiboot kernel image and then verify
the resulting image.

Change-Id: If00e7c85244bc59853c305e42543f34c5fabf356
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://chromium-review.googlesource.com/497933
Reviewed-by: Randall Spangler <rspangler@chromium.org>
2 files changed