image.mk: clean up redundant code related to DEVICE_DTS

It is used by a core build template, so the variable should be
initialized and added to DEVICE_VARS in the core.
Same for DEVICE_DTS_DIR

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/image.mk

@@ -329,11 +329,15 @@ define Device/Init
   SUBPAGESIZE :=
   UBINIZE_OPTS := -E 5
 
+  DEVICE_DTS :=
+  DEVICE_DTS_DIR :=
+
   FILESYSTEMS := $(TARGET_FILESYSTEMS)
 endef
 
 DEFAULT_DEVICE_VARS := \
   DEVICE_NAME KERNEL KERNEL_INITRAMFS KERNEL_INITRAMFS_IMAGE \
+  DEVICE_DTS DEVICE_DTS_DIR \
   UBOOTENV_IN_UBI KERNEL_IN_UBI \
   BLOCKSIZE PAGESIZE SUBPAGESIZE \
   UBINIZE_OPTS