scripts: Adjust EFI script to support an OS

At present the OS disk conflicts with the boot disk used to hold the
app (or payload). Number the OS disk after that.

Signed-off-by: Simon Glass <sjg@chromium.org>

scripts/build_helper.py

@@ -161,7 +161,8 @@ sct_mnt = /mnt/sct
             else:
                 cmd.extend([
                     '-drive',
-                    f'if=virtio,file={os_path},format=raw,id=hd0,readonly=on'])
+                    f'if=virtio,file={os_path},format=raw,id=hd{base_hd},readonly=on'])
+            base_hd += 1
 
         if args.disk:
             for i, d in enumerate(args.disk):