SurfaceFlinger: make the num FBs a board config

Bug: 7283132
Change-Id: I38116f39fc18212f2daab94bbfc3daaf89439fc4
diff --git a/services/surfaceflinger/Android.mk b/services/surfaceflinger/Android.mk
index 0d559e7..5a57697 100644
--- a/services/surfaceflinger/Android.mk
+++ b/services/surfaceflinger/Android.mk
@@ -38,6 +38,10 @@
 	LOCAL_CFLAGS += -DTARGET_DISABLE_TRIPLE_BUFFERING
 endif
 
+ifneq ($(NUM_FRAMEBUFFER_SURFACE_BUFFERS),)
+  LOCAL_CFLAGS += -DNUM_FRAMEBUFFER_SURFACE_BUFFERS=$(NUM_FRAMEBUFFER_SURFACE_BUFFERS)
+endif
+
 LOCAL_SHARED_LIBRARIES := \
 	libcutils \
 	libdl \