hwc2: Use BoardConfig flag to enable/disable HWC2

Change-Id: If2e78d3995898619691cedf281d2f70a4698b682
CRs-fixed: 1026551
diff --git a/common.mk b/common.mk
index 228f742..6b3e2e9 100644
--- a/common.mk
+++ b/common.mk
@@ -2,6 +2,9 @@
 display_top := $(call my-dir)
 
 use_hwc2 := false
+ifeq ($(TARGET_USES_HWC2), true)
+    use_hwc2 := true
+endif
 
 common_includes := $(display_top)/libqdutils
 common_includes += $(display_top)/libqservice