Only pick DTBs from BOARD_PREBUILT_DTBIMAGE_DIR

This change is required to prevent the build system
from picking up DTB files from subdirectories to
BOARD_PREBUILT_DTBIMAGE_DIR.

Test: make bootimage
Bug: 123648105

Change-Id: Ibc2a133f3c9be6362579dc9bf9ca399912b9d8a2
diff --git a/core/Makefile b/core/Makefile
index ea38942..f2988fb 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -840,8 +840,7 @@
 
 ifdef BOARD_PREBUILT_DTBIMAGE_DIR
 INSTALLED_DTBIMAGE_TARGET := $(PRODUCT_OUT)/dtb.img
-$(INSTALLED_DTBIMAGE_TARGET) : $(addprefix $(BOARD_PREBUILT_DTBIMAGE_DIR)/,\
-	$(call find-files-in-subdirs,$(BOARD_PREBUILT_DTBIMAGE_DIR),*.dtb,.))
+$(INSTALLED_DTBIMAGE_TARGET) : $(sort $(wildcard $(BOARD_PREBUILT_DTBIMAGE_DIR)/*.dtb))
 	cat $^ > $@
 endif