Add empty top level Android.mk / CleanSpec.mk files

Needed to allow Android to keep building with a copy of the SELinux
source code checked into external/selinux

Bug: 19963152
Change-Id: I4e192bb6ca5d185e11540aca7253e729d92a00c1
diff --git a/CleanSpec.mk b/CleanSpec.mk
new file mode 100644
index 0000000..1ac5a62
--- /dev/null
+++ b/CleanSpec.mk
@@ -0,0 +1,2 @@
+# This empty CleanSpec.mk file will prevent the build system
+# from descending into subdirs.