hwc: Rename libexternal to serve HDMI as primary or external

This change will make is easier to build new functionality
applicable to both HDMI scenarios.

Change-Id: I417184568cb65a49c2ccb4199aff313ae2ee9b24
diff --git a/common.mk b/common.mk
index 14d8ded..ae6c95a 100644
--- a/common.mk
+++ b/common.mk
@@ -4,7 +4,7 @@
 common_includes += $(LOCAL_PATH)/../libcopybit
 common_includes += $(LOCAL_PATH)/../libqdutils
 common_includes += $(LOCAL_PATH)/../libhwcomposer
-common_includes += $(LOCAL_PATH)/../libexternal
+common_includes += $(LOCAL_PATH)/../libhdmi
 common_includes += $(LOCAL_PATH)/../libqservice
 
 ifeq ($(TARGET_USES_POST_PROCESSING),true)