surfaceflinger: add BoardConfig.mk variable to force 32-bit

Individual devices may need to force surfaceflinger to compile for
32-bit if they don't have 64-bit GL libraries.

Change-Id: I3703b2f8e36e90cf125d9ed5e6318c3506861948
diff --git a/services/surfaceflinger/Android.mk b/services/surfaceflinger/Android.mk
index b2bc550..49a017f 100644
--- a/services/surfaceflinger/Android.mk
+++ b/services/surfaceflinger/Android.mk
@@ -120,6 +120,10 @@
 
 LOCAL_MODULE:= surfaceflinger
 
+ifdef TARGET_32_BIT_SURFACEFLINGER
+LOCAL_32_BIT_ONLY := true
+endif
+
 include $(BUILD_EXECUTABLE)
 
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