Vendor SKU manifests: define env variables properly

In particular, BOARD_SEPOLICY_VERS may be defined in
system/sepolicy/Android.mk, so defer its evaluation.

Test: build with vendor SKU
Bug: 148601741
Change-Id: Iab3f5dcba4edb65deaab38cc2712d22b89e1a83c
diff --git a/target/board/Android.mk b/target/board/Android.mk
index ffc9baf..9edc85c 100644
--- a/target/board/Android.mk
+++ b/target/board/Android.mk
@@ -72,9 +72,9 @@
 GEN := $$(local-generated-sources-dir)/manifest_$(1).xml
 $$(GEN): PRIVATE_SRC_FILES := $$(my_fragment_files)
 $$(GEN): $$(my_fragment_files) $$(HOST_OUT_EXECUTABLES)/assemble_vintf
-	BOARD_SEPOLICY_VERS=$(BOARD_SEPOLICY_VERS) \
-	PRODUCT_ENFORCE_VINTF_MANIFEST=$(PRODUCT_ENFORCE_VINTF_MANIFEST) \
-	PRODUCT_SHIPPING_API_LEVEL=$(PRODUCT_SHIPPING_API_LEVEL) \
+	BOARD_SEPOLICY_VERS=$$(BOARD_SEPOLICY_VERS) \
+	PRODUCT_ENFORCE_VINTF_MANIFEST=$$(PRODUCT_ENFORCE_VINTF_MANIFEST) \
+	PRODUCT_SHIPPING_API_LEVEL=$$(PRODUCT_SHIPPING_API_LEVEL) \
 	$$(HOST_OUT_EXECUTABLES)/assemble_vintf -o $$@ \
 		-i $$(call normalize-path-list,$$(PRIVATE_SRC_FILES))