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
(cherry picked from commit ac948f53ebb3c8823208d81d2021cc006af53c78)

Conflicts:
	config/common.mk