image_signing: Fix missing root key for unibuilds
image_signing: Fix missing root key for unibuilds

For model specific signatures, the root key needs to be copied also for
the development case where the root key can be flashed into the RO
block.

BUG=b:65367246
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: I116850881d3c183b20e7d75e40deb13122f40c7a
Reviewed-on: https://chromium-review.googlesource.com/650546
Commit-Ready: C Shapiro <97eabdef1f1c3aaebe144a81b3e6d8d879701c39@google.com>
Tested-by: C Shapiro <97eabdef1f1c3aaebe144a81b3e6d8d879701c39@google.com>
Reviewed-by: C Shapiro <97eabdef1f1c3aaebe144a81b3e6d8d879701c39@google.com>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot