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/libs.mk b/libs.mk
index 6215990..1650028 100644
--- a/libs.mk
+++ b/libs.mk
@@ -50,6 +50,9 @@
 include $(SRC_PATH_BARE)/vpx_dsp/vpx_dsp.mk
 CODEC_SRCS-yes += $(addprefix vpx_dsp/,$(call enabled,DSP_SRCS))
 
+include $(SRC_PATH_BARE)/vpx_thread/vpx_thread.mk
+CODEC_SRCS-yes += $(addprefix vpx_thread/,$(call enabled,THREAD_SRCS))
+
 ifneq ($(CONFIG_VP8_ENCODER)$(CONFIG_VP8_DECODER),)
   VP8_PREFIX=vp8/
   include $(SRC_PATH_BARE)/$(VP8_PREFIX)vp8_common.mk