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>
(cherry picked from commit 641f6b6c26)

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

@@ -25,7 +25,7 @@ platform_copy_config() {
 	case "$(board_name)" in
 	erlite)
 		mount -t vfat /dev/sda1 /mnt
-		cp -af "$CONF_TAR" "/mnt/$BACKUP_FILE"
+		cp -af "$UPGRADE_BACKUP" "/mnt/$BACKUP_FILE"
 		umount /mnt
 		;;
 	esac