imx6: use DTS_DIR at image build code

Use "$(DTS_DIR)", defined at include/image.mk, instead of
"$(LINUX_DIR)/arch/$(LINUX_KARCH)/boot/dts" in order to generalize and
allow a better Device/* device-tree parameterization (i.e. DEVICE_DTS_DIR
and DTS_DIR).

Signed-off-by: Adrià Llaudet <[email protected]>

target/linux/imx6/image/Makefile

@@ -22,7 +22,7 @@ define Build/boot-overlay
 
 	$(foreach dts,$(DEVICE_DTS), \
 		$(CP) \
-			$(LINUX_DIR)/arch/$(ARCH)/boot/dts/$(dts).dtb \
+			$(DTS_DIR)/$(dts).dtb \
 			[email protected]/$(IMG_PREFIX)-$(dts).dtb; \
 		ln -sf \
 			$(IMG_PREFIX)-$(dts).dtb \