Move multi-threading module functions into vpx_thread folder

This commit moves the primitive multi-threading files from vp9
folder to vpx_thread, which will be accessible by all vpx codec.

Change-Id: Ib51e66e9c69801c10631fab56d35a0c0aaed5883
diff --git a/vpx_thread/vpx_thread.mk b/vpx_thread/vpx_thread.mk
new file mode 100644
index 0000000..0a4a364
--- /dev/null
+++ b/vpx_thread/vpx_thread.mk
@@ -0,0 +1,13 @@
+##
+## Copyright (c) 2015 The WebM project authors. All Rights Reserved.
+##
+##  Use of this source code is governed by a BSD-style license
+##  that can be found in the LICENSE file in the root of the source
+##  tree. An additional intellectual property rights grant can be found
+##  in the file PATENTS.  All contributing project authors may
+##  be found in the AUTHORS file in the root of the source tree.
+##
+
+THREAD_SRCS-yes += vpx_thread.mk
+THREAD_SRCS-yes += vpx_thread.c
+THREAD_SRCS-yes += vpx_thread.h