mvebu: improve reproducibility of ctera firmware

Tar happily stores the username, which my vary on different build machines. Set
it to numerical and 0.

Signed-off-by: Paul Spooren <mail@aparcar.org>

target/linux/mvebu/image/Makefile

@@ -93,6 +93,7 @@ define Build/ctera-firmware
 		> $@.tmp/trailer
 
 	tar $(if $(SOURCE_DATE_EPOCH),--mtime="@$(SOURCE_DATE_EPOCH)") \
+		--numeric-owner --owner=0 --group=0 \
 		-H gnu -C $@.tmp -cf $@.tar header kernel trailer
 
 	# Prepare header and trailer file for fake romdisk
@@ -114,6 +115,7 @@ define Build/ctera-firmware
 		> $@.tmp/trailer
 
 	tar $(if $(SOURCE_DATE_EPOCH),--mtime="@$(SOURCE_DATE_EPOCH)") \
+		--numeric-owner --owner=0 --group=0 \
 		-H gnu -C $@.tmp -rf $@.tar header romdisk trailer
 
 	mv $@.tar $@