commonlib: Add function to hash contents of a CBFS region.

Provide a common routine to hash the contents of a cbfs
region. The cbfs region is hashed in the following order:
1. potential cbfs header at offset 0
2. potential cbfs header retlative offset at cbfs size - 4
3. For each file the metadata of the file.
4. For each non-empty file the data of the file.

BUG=chrome-os-partner:48412
BUG=chromium:445938
BRANCH=None
TEST=Utilized in chromeos cros_bundle_firmware as well as at
      runtime during vboot verification on glados.

Change-Id: Ie1e5db5b8a80d9465e88d3f69f5367d887bdf73f
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/12786
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)

Makefile.inc

@@ -301,6 +301,8 @@ endif
 
 CPPFLAGS_common := -Isrc -Isrc/include -Isrc/commonlib/include -I$(obj)
 CPPFLAGS_common += -Isrc/device/oprom/include
+VB_SOURCE ?= 3rdparty/vboot
+CPPFLAGS_common += -I$(VB_SOURCE)/firmware/include
 CPPFLAGS_common += -include $(src)/include/kconfig.h
 
 CFLAGS_common += -pipe -g -nostdinc