cm versioning: fix certificate check

The variable used does not exist at this stage of the build. Switch
to PRODUCT_DEFAULT_DEV_CERTIFICATE, which does exist when in use.

Change-Id: Icb3bbaa437e68b96e1ce05a09f6525cd1ddb9a6a
merge remote

Change-Id: I8ee0267e3be59263be71c0e96d9f6a25ab5d7fb1