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 branch 'cm-11.0' of https://github.com/CyanogenMod/android_vendor_cm into cm-11.0