Allow maintenance versions to be ignored

CyanogenMod builds shouldn't show maintenance versions.  Allow the
environment to override this behavior instead

Change-Id: I77c924a13122c8e1d9af14501e46bc532f7aefbd

config/common.mk

@@ -233,7 +233,12 @@ DEVICE_PACKAGE_OVERLAYS += vendor/cm/overlay/common
 
 PRODUCT_VERSION_MAJOR = 13
 PRODUCT_VERSION_MINOR = 0
-PRODUCT_VERSION_MAINTENANCE = 1
+ifeq ($(TARGET_VENDOR_SHOW_MAINTENANCE_VERSION),true)
+    PRODUCT_VERSION_MAINTENANCE := 1
+else
+    # 0 is ignored
+    PRODUCT_VERSION_MAINTENANCE := 0
+endif
 
 # Set CM_BUILDTYPE from the env RELEASE_TYPE, for jenkins compat