add vb2api_get_pcr_digest

this api allows firmware to get the digest indicating boot mode status.

BUG=chromium:451609
TEST=VBOOT2=1 make run2tests
BRANCH=tot

Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
Change-Id: Idca7bc5f6aed947689ad7cf219805aad35047c7d
Reviewed-on: https://chromium-review.googlesource.com/244542
9 files changed