treewide: don't hardcode "sysupgrade.tgz" file name

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 bf390478727ac5f4f9d6fb684de48b8150bcec67)