layerscape: make boot filesystem reproducible

Set timestamp option for make_ext4fs to make boot filesystem
reproducible.

Signed-off-by: Daniel Golle <[email protected]>

target/linux/layerscape/image/Makefile

@@ -36,7 +36,9 @@ endef
 define Build/ls-append-kernel
 	mkdir -p [email protected] && \
 	cp $(IMAGE_KERNEL) [email protected]/fitImage && \
-	make_ext4fs -J -L kernel -l "$(LS_SD_KERNELPART_SIZE)M" "[email protected]" "[email protected]" && \
+	make_ext4fs -J -L kernel -l "$(LS_SD_KERNELPART_SIZE)M" \
+		$(if $(SOURCE_DATE_EPOCH),-T $(SOURCE_DATE_EPOCH)) \
+		"[email protected]" "[email protected]" && \
 	dd [email protected] >> $@ && \
 	rm -rf [email protected] && \
 	rm -f [email protected]