[unibuild] Support for unibuild firmware signing

For design context, see go/cros-unibuild-signing

This adds support for multiple, shared firmware images from a unified
build that needs to be signed with different OEM specific keys.

It uses a signer_config.csv file (that is generated by pack_firmware.py)
to determine which images need to be signed with which keys.

BUG=b:64842314
TEST=./build_image --board=coral dev
&& ./mod_image_for_recovery.sh --board=coral
&& ~/trunk/src/platform/vboot_reference/scripts/image_signing/sign_official_build.sh
recovery ../build/images/coral/latest/recovery_image.bin
../platform/vboot_reference/tests/devkeys
BRANCH=None

Change-Id: Id3711bbe73dfe652184bc046b5f642c30b8d1627
Reviewed-on: https://chromium-review.googlesource.com/626718
Commit-Ready: C Shapiro <shapiroc@google.com>
Tested-by: C Shapiro <shapiroc@google.com>
Reviewed-by: C Shapiro <shapiroc@google.com>
1 file changed