sign_official_build: fix support for signing standalone accessories

My previous patch using futility to re-sign standalone accessory
firmware images had a mistake in the key directory path : fix it.

Also add RSA-2048 'accessory' keys for signer unit testing.

BRANCH=smaug, samus
BUG=chrome-os-partner:46635
TEST=run cros-signing unittests (./signing_unittests.py)

Change-Id: Ia2f641c85337c67f81968be4730643a6ad5f22cf
Reviewed-on: https://chromium-review.googlesource.com/309530
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
4 files changed