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

@@ -8,8 +8,8 @@ move_config() {
 
 	if export_bootdevice && export_partdevice partdev 1; then
 		if mount -t vfat -o rw,noatime "/dev/$partdev" /mnt; then
-			if [ -f /mnt/sysupgrade.tgz ]; then
-				mv -f /mnt/sysupgrade.tgz /
+			if [ -f "/mnt/$BACKUP_FILE" ]; then
+				mv -f "/mnt/$BACKUP_FILE" /
 			fi
 			umount /mnt
 		fi