cm versioning: Empty keys == default keys

Don't assume the path to the signing key is the default, it can
be empty and have the same meaning...

Change-Id: Id545d6436e9f21d6b62eec5a317ea7665190f104
diff --git a/config/common.mk b/config/common.mk
index 91dcd7d..3dc5cac 100644
--- a/config/common.mk
+++ b/config/common.mk
@@ -297,6 +297,7 @@
 
 CM_DISPLAY_VERSION := $(CM_VERSION)
 
+ifneq ($(DEFAULT_SYSTEM_DEV_CERTIFICATE),)
 ifneq ($(DEFAULT_SYSTEM_DEV_CERTIFICATE),build/target/product/security/testkey)
   ifneq ($(CM_BUILDTYPE), UNOFFICIAL)
     ifndef TARGET_VENDOR_RELEASE_BUILD_ID
@@ -311,6 +312,7 @@
     CM_DISPLAY_VERSION=$(PRODUCT_VERSION_MAJOR).$(PRODUCT_VERSION_MINOR)$(TARGET_VENDOR_RELEASE_BUILD_ID)
   endif
 endif
+endif
 
 PRODUCT_PROPERTY_OVERRIDES += \
   ro.cm.display.version=$(CM_DISPLAY_VERSION)