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/apm821xx/base-files/lib/preinit/79_move_config

@@ -4,13 +4,14 @@ BOOTPART=/dev/sda1
 
 move_config() {
 	. /lib/functions.sh
+	. /lib/upgrade/common.sh
 
 	case "$(board_name)" in
 	wd,mybooklive)
 		if [ -b $BOOTPART ]; then
 			mkdir -p /boot
 			mount -t ext4 -o rw,noatime $BOOTPART /boot
-			[ -f /boot/sysupgrade.tgz ] && mv -f /boot/sysupgrade.tgz /
+			[ -f "/boot/$BACKUP_FILE" ] && mv -f "/boot/$BACKUP_FILE" /
 		fi
 		;;
 	*)