configstore: add new configuration item

This change adds to configstore@1.0 HAL a new configuration item
for surfaceflinger; useTripleFramebuffer()

In the default implementation, useTripleFramebuffer() will return true
only if the build flag NUM_FRAMEBUFFER_SURFACE_BUFFERS is defined as 3.

Bug: 34723491
Test: build & run. log message "SurfaceFlinger FrameBuffer: ?"
      should be seen.

Change-Id: Iac97e25cd69717c73a135f6b2a44538124544987
diff --git a/configstore/1.0/default/surfaceflinger.mk b/configstore/1.0/default/surfaceflinger.mk
index 42fa191..5a946f4 100644
--- a/configstore/1.0/default/surfaceflinger.mk
+++ b/configstore/1.0/default/surfaceflinger.mk
@@ -4,3 +4,7 @@
 ifneq ($(VSYNC_EVENT_PHASE_OFFSET_NS),)
     LOCAL_CFLAGS += -DVSYNC_EVENT_PHASE_OFFSET_NS=$(VSYNC_EVENT_PHASE_OFFSET_NS)
 endif
+
+ifeq ($(NUM_FRAMEBUFFER_SURFACE_BUFFERS),3)
+    LOCAL_CFLAGS += -DUSE_TRIPLE_FRAMEBUFFER
+endif