Move sdk staging dir to a product-specific dir

So we can do incremental build across different SDK products.

Change-Id: If3f082e4b92d00e7ccda980610e1d967d44da622
diff --git a/core/Makefile b/core/Makefile
index bb3e64e..fbd04cc 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1456,7 +1456,7 @@
 # The SDK
 
 # The SDK includes host-specific components, so it belongs under HOST_OUT.
-sdk_dir := $(HOST_OUT)/sdk
+sdk_dir := $(HOST_OUT)/sdk/$(TARGET_PRODUCT)
 
 # Build a name that looks like:
 #