android: add note about command line defines and config.h

Cc: Chih-Wei Huang <cwhuang@android-x86.org>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
diff --git a/Android.common.mk b/Android.common.mk
index ffe9219..71f14ec 100644
--- a/Android.common.mk
+++ b/Android.common.mk
@@ -1,3 +1,4 @@
+# XXX: Consider moving these to config.h analogous to autoconf.
 LOCAL_CFLAGS += \
 	-DHAVE_VISIBILITY=1 \
 	-DHAVE_LIBDRM_ATOMIC_PRIMITIVES=1