base-files: pass "save_config" option to the "sysupgrade" method

This explicitly lets stage2 know if config should be preserved.

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

package/base-files/files/sbin/sysupgrade

@@ -369,6 +369,9 @@ else
 	ubus call system sysupgrade "{
 		\"prefix\": $(json_string "$RAM_ROOT"),
 		\"path\": $(json_string "$IMAGE"),
-		\"command\": $(json_string "$COMMAND")
+		\"command\": $(json_string "$COMMAND"),
+		\"options\": {
+			\"save_config\": $SAVE_CONFIG
+		}
 	}"
 fi