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

@@ -54,7 +54,7 @@ platform_check_image() {
 platform_copy_config_emmc() {
 	mkdir -p /recovery
 	mount -o rw,noatime /dev/mmcblk0p1 /recovery
-	cp -af "$CONF_TAR" "/recovery/$BACKUP_FILE"
+	cp -af "$UPGRADE_BACKUP" "/recovery/$BACKUP_FILE"
 	sync
 	umount /recovery
 }