vboot2: move verify digest to 2common

This removes code duplicated between 2common.c and 2rsa.c.  This is in
preparation for adding new unsigned hash algorithms.

BUG=chromium:423882
BRANCH=none
TEST=VBOOT2=1 make -j runtests

Change-Id: Ic9c542ae14d3b7f786129c1d52f8963847a94fb8
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/224780
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
8 files changed