treewide: sysupgrade: use $UPGRADE_BACKUP to check for backup

Now that $UPGRADE_BACKUP is set conditionally there is no need to check
the $UPGRADE_OPT_SAVE_CONFIG anymore. All conditions can be simplified.

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

target/linux/imx6/base-files/lib/upgrade/platform.sh

@@ -75,7 +75,7 @@ platform_pre_upgrade() {
 
 	case "$board" in
 	apalis*)
-		[ "$UPGRADE_OPT_SAVE_CONFIG" -eq 0 ] && {
+		[ -z "$UPGRADE_BACKUP" ] && {
 			jffs2reset -y
 			umount /overlay
 		}