build: Use stable file ordering in sysupgrade.tar

The file ordering in the current sysupgrade depends on the order of file in
a filesystem. This is often already in a sane (alphabetical order) but this
is not always the case. For example, the OpenWrt build servers return a
different ordering.

This breaks the reproducibility of the sysupgrade tarballs significantly
and also resulted in images which cannot be used for upgrades on devices
like the OpenMesh A42/A62.

Signed-off-by: Sven Eckelmann <sven@narfation.org>

scripts/sysupgrade-tar.sh

@@ -61,7 +61,7 @@ if [ -n "$SOURCE_DATE_EPOCH" ]; then
 	mtime="--mtime=@${SOURCE_DATE_EPOCH}"
 fi
 
-(cd "$tmpdir"; tar cvf sysupgrade.tar sysupgrade-${board} ${mtime})
+(cd "$tmpdir"; tar --sort=name -cvf sysupgrade.tar sysupgrade-${board} ${mtime})
 err="$?"
 if [ -e "$tmpdir/sysupgrade.tar" ]; then
 	cp "$tmpdir/sysupgrade.tar" "$outfile"