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/omap/base-files/lib/upgrade/platform.sh

@@ -30,7 +30,7 @@ platform_copy_config() {
 
 	if export_partdevice partdev 1; then
 		mount -t vfat -o rw,noatime "/dev/$partdev" /mnt
-		cp -af "$CONF_TAR" "/mnt/$BACKUP_FILE"
+		cp -af "$UPGRADE_BACKUP" "/mnt/$BACKUP_FILE"
 		umount /mnt
 	fi
 }