Move from WITH_ART to PRODUCT_RUNTIMES

Change-Id: I3a9217b4aafc1f59d095169deb9eaeae17320505
diff --git a/buildspec.mk.default b/buildspec.mk.default
index 397194c..6f9e755 100644
--- a/buildspec.mk.default
+++ b/buildspec.mk.default
@@ -104,9 +104,15 @@
 #ENABLE_SVG:=false
 endif
 
-# To enable ART define WITH_ART:=true
-ifndef WITH_ART
-#WITH_ART:=true
+# OVERRIDE_RUNTIMES allows you to locally override PRODUCT_RUNTIMES.
+#
+# To only build ART, use "runtime_libart_default"
+# To use Dalvik but also include ART, use "runtime_libdvm_default runtime_libart"
+# To use ART but also include Dalvik, use "runtime_libart_default runtime_libdvm"
+ifndef OVERRIDE_RUNTIMES
+#OVERRIDE_RUNTIMES:=runtime_libart_default
+#OVERRIDE_RUNTIMES:=runtime_libdvm_default runtime_libart
+#OVERRIDE_RUNTIMES:=runtime_libart_default runtime_libdvm
 endif
 
 # when the build system changes such that this file must be updated, this