Move the prebuilt NDKs to their own git project prebuilts/ndk/.

Change-Id: If85986da436d412cff139af1fd8426e420bec121
diff --git a/core/config.mk b/core/config.mk
index 3e10c93..27a9f4b 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -340,7 +340,7 @@
 # ###############################################################
 
 HISTORICAL_SDK_VERSIONS_ROOT := $(TOPDIR)prebuilts/sdk
-HISTORICAL_NDK_VERSIONS_ROOT := $(TOPDIR)prebuilt/ndk
+HISTORICAL_NDK_VERSIONS_ROOT := $(TOPDIR)prebuilts/ndk
 
 # Historical SDK version N is stored in $(HISTORICAL_SDK_VERSIONS_ROOT)/N.
 # The 'current' version is whatever this source tree is.