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)

package/base-files/files/lib/upgrade/common.sh

@@ -2,6 +2,8 @@
 
 RAM_ROOT=/tmp/root
 
+export BACKUP_FILE=sysupgrade.tgz	# file extracted by preinit
+
 [ -x /usr/bin/ldd ] || ldd() { LD_TRACE_LOADED_OBJECTS=1 $*; }
 libs() { ldd $* 2>/dev/null | sed -r 's/(.* => )?(.*) .*/\2/'; }