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>
(cherry picked from commit bf39047872)

target/linux/octeon/base-files/lib/preinit/79_move_config

@@ -3,11 +3,12 @@
 
 move_config() {
 	. /lib/functions.sh
+	. /lib/upgrade/common.sh
 
 	case "$(board_name)" in
 		erlite)
 			mount -t vfat /dev/sda1 /mnt
-			[ -f /mnt/sysupgrade.tgz ] && mv -f /mnt/sysupgrade.tgz /
+			[ -f "/mnt/$BACKUP_FILE" ] && mv -f "/mnt/$BACKUP_FILE" /
 			umount /mnt
 			;;
 	esac