Allow for the DTB image to be built by the Android build system.

The current solution expects BOARD_PREBUILT_DTBIMAGE_DIR to
contain prebuilt DTB files that are concatenated by the build system
to create $OUT/dtb.img. In order to accommodate devices that build
the dtb image locally, make boot.img creation depend on $OUT/dtb.img
only when BOARD_PREBUILT_DTBIMAGE_DIR is undefined.

Bug: 133161451
Test: Build with BOARD_PREBUILT_DTBIMAGE_DIR undefined and verify
using unpack_bootimg.py that $OUT/dtb.img was included in boot.img.

Change-Id: Iae2c634ccdc1d83589b26d382882f75fb8565a31
diff --git a/core/board_config.mk b/core/board_config.mk
index 2580a33..c3c6646 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -493,6 +493,12 @@
   endif
 endif
 
+ifdef BOARD_PREBUILT_DTBIMAGE_DIR
+  ifneq ($(BOARD_INCLUDE_DTB_IN_BOOTIMG),true)
+    $(error BOARD_PREBUILT_DTBIMAGE_DIR with 'BOARD_INCLUDE_DTB_IN_BOOTIMG != true' is not supported)
+  endif
+endif
+
 # Check BOARD_VNDK_VERSION
 define check_vndk_version
   $(eval vndk_path := prebuilts/vndk/v$(1)) \