sign_official_build: add a fallback key for standalone accessories

The standalone accessories are using a key name like this:
key_<product>.pem or key_<product>.vbprik2
when it doesn't exist, fallback using key.pem or key.vbprik2.

BRANCH=none
BUG=chrome-os-partner:47557
TEST=manual:
./scripts/image_signing/sign_official_build.sh accessory_usbpd ../ec/build/zinger/ec.bin tests/devkeys-acc /tmp/ec-zinger.TEST.SIGNED.bin
./scripts/image_signing/sign_official_build.sh accessory_rwsig ../ec/private/build/hadoken/keyboard_app.bin tests/devkeys-acc /tmp/ec-hadoken.TEST.SIGNED.bin
./scripts/image_signing/sign_official_build.sh accessory_rwsig ../ec/private/build/hadoken/keyboard_app.bin /tmp /tmp/ec-hadoken.TEST.SIGNED.bin

Change-Id: I68863664bdb9da1695e91b1986f3a0148af7da26
Reviewed-on: https://chromium-review.googlesource.com/312836
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/image_signing/sign_official_build.sh b/scripts/image_signing/sign_official_build.sh
index aa3dc24..a3f7c98 100755
--- a/scripts/image_signing/sign_official_build.sh
+++ b/scripts/image_signing/sign_official_build.sh
@@ -883,10 +883,16 @@
   sign_update_payload ${INPUT_IMAGE} ${KEY_DIR} ${OUTPUT_IMAGE}
 elif [[ "${TYPE}" == "accessory_usbpd" ]]; then
   KEY_NAME="${KEY_DIR}/key_$(basename $(dirname ${INPUT_IMAGE}))"
+  if [[ ! -e "${KEY_NAME}.pem" ]]; then
+    KEY_NAME="${KEY_DIR}/key"
+  fi
   cp "${INPUT_IMAGE}" "${OUTPUT_IMAGE}"
   futility sign --type usbpd1 --pem "${KEY_NAME}.pem" "${OUTPUT_IMAGE}"
 elif [[ "${TYPE}" == "accessory_rwsig" ]]; then
   KEY_NAME="${KEY_DIR}/key_$(basename $(dirname ${INPUT_IMAGE}))"
+  if [[ ! -e "${KEY_NAME}.vbprik2" ]]; then
+    KEY_NAME="${KEY_DIR}/key"
+  fi
   cp "${INPUT_IMAGE}" "${OUTPUT_IMAGE}"
   futility sign --type rwsig --prikey "${KEY_NAME}.vbprik2" "${OUTPUT_IMAGE}"
 else