Move sdk staging dir to a product-specific dir

So we can do incremental build across different SDK products.

(cherry picked from commit d7af176199)

Change-Id: I30ebed5328123629519a64c793f94fc8d5aa09a8

core/Makefile

@@ -1510,7 +1510,7 @@ endif
 # 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:
 #