treewide: when copying a backup file always specify dest name

$CONF_TAR shouldn't be assumed to always point to the sysupgrade.tgz.
This change makes code more generic and allows refactoring $CONF_TAR.

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

target/linux/x86/base-files/lib/upgrade/platform.sh

@@ -39,7 +39,7 @@ platform_copy_config() {
 
 	if export_partdevice partdev 1; then
 		mount -t ext4 -o rw,noatime "/dev/$partdev" /mnt
-		cp -af "$CONF_TAR" /mnt/
+		cp -af "$CONF_TAR" "/mnt/$BACKUP_FILE"
 		umount /mnt
 	fi
 }