image: make kernel UBIFS images reproducible

The temporary directory used to create a kernel UBIFS image and its
kernel file receive wall-clock mtimes. mkfs.ubifs preserves these
times, making images differ between builds.

Set their timestamps to SOURCE_DATE_EPOCH before invoking mkfs.ubifs.

Signed-off-by: Robert Marko <robimarko@gmail.com>

include/image-commands.mk

@@ -30,6 +30,9 @@ endef
 define Build/package-kernel-ubifs
 	mkdir $@.kernelubifs
 	cp $@ $@.kernelubifs/kernel
+	$(if $(SOURCE_DATE_EPOCH), \
+		touch -hcd "@$(SOURCE_DATE_EPOCH)" \
+		$@.kernelubifs $@.kernelubifs/kernel)
 	$(STAGING_DIR_HOST)/bin/mkfs.ubifs \
 		$(KERNEL_UBIFS_OPTS) \
 		-r $@.kernelubifs $@