Move prebuilt SDKs to their own git project.

Change-Id: Ie6e9614e43bf82b8dd1f8702968be40cfb76b6c5
diff --git a/core/config.mk b/core/config.mk
index d486ed3..3e10c93 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -339,7 +339,7 @@
 # For use with the LOCAL_SDK_VERSION variable for include $(BUILD_PACKAGE)
 # ###############################################################
 
-HISTORICAL_SDK_VERSIONS_ROOT := $(TOPDIR)prebuilt/sdk
+HISTORICAL_SDK_VERSIONS_ROOT := $(TOPDIR)prebuilts/sdk
 HISTORICAL_NDK_VERSIONS_ROOT := $(TOPDIR)prebuilt/ndk
 
 # Historical SDK version N is stored in $(HISTORICAL_SDK_VERSIONS_ROOT)/N.