image_signing: add log message when signing ARC++
image_signing: add log message when signing ARC++

Instead of only logging when ARC++ is not present, also report when we
found an ARC++ image about to be re-signed.

BUG=None
TEST=See info message when running sign_official_build.sh
BRANCH=None

Change-Id: I0d983d38048c4b8dace51e4ea25e23c7cf1da3d7
Reviewed-on: https://chromium-review.googlesource.com/942021
Commit-Ready: Nicolas Norvez <3001a977f47a2bc94ec05551872e2ce88b323556@chromium.org>
Tested-by: Nicolas Norvez <3001a977f47a2bc94ec05551872e2ce88b323556@chromium.org>
Reviewed-by: Victor Hsieh <e50b749460aecb2d84d38a0c40b84707be8ae902@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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