multiboot: Support booting multiboot kernels

Enable support for loading and booting multiboot format kernels
that are properly signed.

This is useful for testing other boot methods or kernels without
using the legacy mode firmware image.

BUG=b:38040849
BRANCH=none
TEST=manual testing with multiboot format kernel

Change-Id: Ib7407cd02d2441b61c73e7ce803a9b397df083f0
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://chromium-review.googlesource.com/498011
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
8 files changed