treewide: use new procd sysupgrade $UPGRADE_BACKUP variable

It's a variable set by procd that should replace hardcoded
/tmp/sysupgrade.tgz.

This change requires the most recent procd with the commit 0f3c136
("sysupgrade: set UPGRADE_BACKUP env variable").

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

target/linux/ipq806x/base-files/lib/upgrade/zyxel.sh

@@ -60,7 +60,7 @@ zyxel_do_flash() {
 	mkdir /tmp/new_root
 	mount -t ext4 $loopdev /tmp/new_root && {
 		echo "Saving config to rootfs_data at position ${offset}."
-		cp -v "$CONF_TAR" "/tmp/new_root/$BACKUP_FILE"
+		cp -v "$UPGRADE_BACKUP" "/tmp/new_root/$BACKUP_FILE"
 		umount /tmp/new_root
 	}