cbfstool: Add 'hashcbfs' command to compute hash of CBFS region.

cbfstool: Add 'hashcbfs' command to compute hash of CBFS region.

For the purposes of maintaining integrity of a CBFS allow one to
hash a CBFS over a given region. The hash consists of all file
metadata and non-empty file data. The resulting digest is saved
to the requested destination region.

BUG=chrome-os-partner:48412
BUG=chromium:445938
BRANCH=None
TEST=Integrated with glados chrome os build. vboot verification
     works using the same code to generate the hash in the tooling
     as well as at runtime on the board in question.

Change-Id: Ib0d6bf668ffd6618f5f73e1217bdef404074dbfc
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/12790
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>