treewide: don't hardcode "sysupgrade.tgz" file name

1) Add BACKUP_FILE and use it when copying an archive to be restored
   after sysupgrade (on the next preinit).
2) Use CONF_TAR for copying backup prepared by the /sbin/sysupgrade

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/ipq806x/base-files/lib/upgrade/zyxel.sh

@@ -60,7 +60,7 @@ zyxel_do_flash() {
 	mkdir /tmp/new_root
 	mount -t ext4 $loopdev /tmp/new_root && {
 		echo "Saving config to rootfs_data at position ${offset}."
-		cp -v /tmp/sysupgrade.tgz /tmp/new_root/
+		cp -v "$CONF_TAR" /tmp/new_root/
 		umount /tmp/new_root
 	}