mvebu: make boot filesystem reproducible

Set timestamp option for make_ext4fs to make boot filesystem
reproducible.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/mvebu/image/Makefile

@@ -34,7 +34,9 @@ define Build/boot-img-ext4
 	$(foreach dts,$(DEVICE_DTS), $(CP) $(KDIR)/image-$(dts).dtb $@.boot/$(dts).dtb;)
 	$(CP) $(IMAGE_KERNEL) $@.boot/$(KERNEL_NAME)
 	-$(CP) $@-boot.scr $@.boot/boot.scr
-	make_ext4fs -J -L kernel -l $(CONFIG_TARGET_KERNEL_PARTSIZE)M $@.bootimg $@.boot
+	make_ext4fs -J -L kernel -l $(CONFIG_TARGET_KERNEL_PARTSIZE)M \
+		$(if $(SOURCE_DATE_EPOCH),-T $(SOURCE_DATE_EPOCH)) \
+		$@.bootimg $@.boot
 endef
 
 define Build/buffalo-kernel-jffs2