build: add UBOOT_PATH to DEFAULT_DEVICE_VARS and set a default

The helper shared Build/append-uboot in include/image-commands.mk
uses it, so include this variable in DEFAULT_DEVICE_VARS.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

include/image.mk

@@ -383,6 +383,8 @@ define Device/Init
   IMAGE_METADATA :=
 
   FILESYSTEMS := $(TARGET_FILESYSTEMS)
+
+  UBOOT_PATH :=  $(STAGING_DIR_IMAGE)/uboot-$(1)
 endef
 
 DEFAULT_DEVICE_VARS := \
@@ -390,7 +392,7 @@ DEFAULT_DEVICE_VARS := \
   CMDLINE UBOOTENV_IN_UBI KERNEL_IN_UBI BLOCKSIZE PAGESIZE SUBPAGESIZE \
   VID_HDR_OFFSET UBINIZE_OPTS UBINIZE_PARTS MKUBIFS_OPTS DEVICE_DTS \
   DEVICE_DTS_CONFIG DEVICE_DTS_DIR BOARD_NAME UIMAGE_NAME SUPPORTED_DEVICES \
-  IMAGE_METADATA KERNEL_ENTRY KERNEL_LOADADDR
+  IMAGE_METADATA KERNEL_ENTRY KERNEL_LOADADDR UBOOT_PATH
 
 define Device/ExportVar
   $(1) : $(2):=$$($(2))