build: fix generating dtb with / in DEVICE_DTS

Fixes layerscape build error

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

include/image.mk

@@ -440,7 +440,7 @@ define Device/Build/dtb
   ifndef BUILD_DTS_$(1)
   BUILD_DTS_$(1) := 1
   $(KDIR)/image-$(1).dtb: FORCE
-	$(call Image/BuildDTB,$(strip $(2))/$(1).dts,$$@)
+	$(call Image/BuildDTB,$(strip $(2))/$(strip $(3)).dts,$$@)
 
   image_prepare: $(KDIR)/image-$(1).dtb
   endif
@@ -450,8 +450,9 @@ endif
 
 define Device/Build/kernel
   $$(eval $$(foreach dts,$$(DEVICE_DTS), \
-	$$(call Device/Build/dtb,$$(dts), \
-		$$(if $$(DEVICE_DTS_DIR),$$(DEVICE_DTS_DIR),$$(DTS_DIR)) \
+	$$(call Device/Build/dtb,$$(notdir $$(dts)), \
+		$$(if $$(DEVICE_DTS_DIR),$$(DEVICE_DTS_DIR),$$(DTS_DIR)), \
+		$$(dts) \
 	) \
   ))