Support custom sdk.atree files.

Bug: 12683047
Change-Id: I79ca1eff81e947d03c50759d47426487e48435d0
diff --git a/core/product.mk b/core/product.mk
index cf6fb9a..3b6b27e 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -86,6 +86,7 @@
     PRODUCT_PACKAGE_OVERLAYS \
     DEVICE_PACKAGE_OVERLAYS \
     PRODUCT_TAGS \
+    PRODUCT_SDK_ATREE_FILES \
     PRODUCT_SDK_ADDON_NAME \
     PRODUCT_SDK_ADDON_COPY_FILES \
     PRODUCT_SDK_ADDON_COPY_MODULES \