Add PRODUCT_VALIDATION_CHECKS

This is a new mechanism for asserting properties about your product
config. See the documentation in product_validation_checks.mk for
more information.

Test: Manually
Change-Id: I698dea899441f3773f839ea2ba1a2a6cfe59b57b
diff --git a/core/envsetup.mk b/core/envsetup.mk
index 7ddbf32..cfb8a66 100644
--- a/core/envsetup.mk
+++ b/core/envsetup.mk
@@ -371,6 +371,8 @@
 TARGET_BUILD_TYPE := release
 endif
 
+include $(BUILD_SYSTEM)/product_validation_checks.mk
+
 # ---------------------------------------------------------------
 # figure out the output directories