ensure an empty BUILD_DIR variable is passed down u-boot's makefile

SVN-Revision: 23832

tools/mkimage/Makefile

@@ -21,7 +21,7 @@ include $(INCLUDE_DIR)/host-build.mk
 define Host/Compile
 	rm -f $(HOST_BUILD_DIR)/tools/.depend
 	touch $(HOST_BUILD_DIR)/include/config.{h,mk} $(HOST_BUILD_DIR)/include/autoconf.mk{.dep,}
-	$(MAKE) -C $(HOST_BUILD_DIR) SUBDIRS=tools BIN_FILES-y="mkimage" tools
+	$(MAKE) -C $(HOST_BUILD_DIR) BUILD_DIR= SUBDIRS=tools BIN_FILES-y="mkimage" tools
 endef
 
 define Host/Install