base-files: upgrade: correctly handle nand_do_upgrade argument passed from preupgrade

Fixes: 30f61a34b4 "base-files: always use staged sysupgrade"
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

package/base-files/files/lib/upgrade/common.sh

@@ -211,7 +211,7 @@ default_do_upgrade() {
 do_upgrade_stage2() {
 	v "Performing system upgrade..."
 	if [ -n "$do_upgrade" ]; then
-		$do_upgrade "$IMAGE"
+		eval "$do_upgrade"
 	elif type 'platform_do_upgrade' >/dev/null 2>/dev/null; then
 		platform_do_upgrade "$IMAGE"
 	else