Fix non-existing modules check

This variable was accidentally changed to refer to the
shorthand PRODUCT_XYZ instead of PRODUCTS.<mk>.PRODUCT_XYZ
in a previous mass-migration, but that shorthand didn't
exist for these variables. Add it in.

Also whitelist the offender introduced since it was broken.

Test: lunch generic; m nothing
Change-Id: I29dd73c29796743e770afa0c2ceb4f8583dc6e1e

core/product.mk

@@ -541,10 +541,15 @@ $(call readonly-variables,$(_readonly_late_variables))
 endef
 
 #
-# Strip the variables in _product_strip_var_list
+# Strip the variables in _product_var_list and a few build-system
+# internal variables, and assign the ones for the current product
+# to a shorthand that is more convenient to read from elsewhere.
 #
 define strip-product-vars
-$(foreach v,$(_product_var_list), \
+$(foreach v,\
+  $(_product_var_list) \
+    PRODUCT_ENFORCE_PACKAGES_EXIST \
+    PRODUCT_ENFORCE_PACKAGES_EXIST_WHITELIST, \
   $(eval $(v) := $(strip $(PRODUCTS.$(INTERNAL_PRODUCT).$(v)))) \
 )
 endef