cros_bundle_firmware fix for complete coreboot images

coreboot builds images with ME firmware now. Fix cros_bundle_firmware
to cope with that.
Previously the coreboot ebuild had to provide a skeleton binary that
contained the Intel Firmware Descriptor and ME firmware, plus a binary
containing coreboot. Now there is only one binary that is used.
In order to provide a minimally intrusive change, use the new coreboot
binary as skeleton file, and carve out the coreboot portion from the
skeleton, after making all changes (like adding a payload etc) to the
full ("skeleton") image.

CQ-DEPEND=CL:*23708 CL:*23709 CL:*25948
CQ-DEPEND=CL:*25952 CL:*25953 CL:*25954
CQ-DEPEND=CL:*25955 CL:*25956 CL:*25957
CQ-DEPEND=CL:30616

BUG=none
TEST=emerge-link chromeos-coreboot-link chromeos-bootimage
     flash resulting image to Link, observe that it is still booting.
BRANCH=none

Change-Id: Idbd8dca54f6c50150f3bca1cabf7e5e3eb760c71
Reviewed-on: https://gerrit.chromium.org/gerrit/34487
Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
Tested-by: Stefan Reinauer <reinauer@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
Commit-Ready: Stefan Reinauer <reinauer@chromium.org>
1 file changed