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/mvebu/base-files/lib/preinit/79_move_config

@@ -18,7 +18,7 @@ move_config() {
 		esac
 		mkdir -p /boot
 		mount -o rw,noatime "/dev/$partdev" /boot
-		[ -f /boot/sysupgrade.tgz ] && mv -f /boot/sysupgrade.tgz /
+		[ -f "/boot/$BACKUP_FILE" ] && mv -f "/boot/$BACKUP_FILE" /
 		umount /boot
 	fi
 }