treewide: replace remaining (not working now) $SAVE_CONFIG uses

This var has been replaced by the $UPGRADE_OPT_UPGRADE_OPT_SAVE_CONFIG

Fixes: f25d164aca ("base-files: pass "save_config" option to the "sysupgrade" method")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 5797fe84a3)

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

@@ -78,7 +78,7 @@ platform_pre_upgrade() {
 
 	case "$board" in
 	apalis*)
-		[ "$SAVE_CONFIG" -eq 0 ] && {
+		[ "$UPGRADE_OPT_SAVE_CONFIG" -eq 0 ] && {
 			jffs2reset -y
 			umount /overlay
 		}