Build libbt only if BOARD_HAVE_BLUETOOTH_MRVL is defined

Change-Id: I2b03a77416d7347fc68c6771eae84268ae38115d
Signed-off-by: Toshi Kikuchi <toshik@google.com>
diff --git a/Android.mk b/Android.mk
index d34dc85..948a037 100644
--- a/Android.mk
+++ b/Android.mk
@@ -1 +1,3 @@
+ifneq ($(BOARD_HAVE_BLUETOOTH_MRVL),)
 include $(call all-named-subdir-makefiles,libbt-vendor)
+endif