HWC2: Hook up to BoardConfig variable

Uses TARGET_USES_HWC2 to enable the HWC2 code path in SurfaceFlinger

Bug: 27211042
Change-Id: I7b7e3e3f7a918e74df0be27418196c81b4332564
diff --git a/services/surfaceflinger/Android.mk b/services/surfaceflinger/Android.mk
index d654b17..2a8fa4a 100644
--- a/services/surfaceflinger/Android.mk
+++ b/services/surfaceflinger/Android.mk
@@ -45,8 +45,7 @@
 LOCAL_CFLAGS := -DLOG_TAG=\"SurfaceFlinger\"
 LOCAL_CFLAGS += -DGL_GLEXT_PROTOTYPES -DEGL_EGLEXT_PROTOTYPES
 
-USE_HWC2 := false
-ifeq ($(USE_HWC2),true)
+ifeq ($(TARGET_USES_HWC2),true)
     LOCAL_CFLAGS += -DUSE_HWC2
     LOCAL_SRC_FILES += \
         SurfaceFlinger.cpp \