sandbox: only do sandboxfs for hostfs interface

Only do sandbox filesystem access when using the hostfs device
interface, rather then falling back to it in all cases. This prevents
confusion situations due to the fallback being taken rather then an
unsupported error being raised.
Signed-off-by: NSjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: NSimon Glass <sjg@chromium.org>
Acked-by: NSimon Glass <sjg@chromium.org>

fs/sandbox/sandboxfs.c

@@ -10,7 +10,11 @@
 
 int sandbox_fs_set_blk_dev(block_dev_desc_t *rbdd, disk_partition_t *info)
 {
-	return 0;
+	/*
+	 * Only accept a NULL block_dev_desc_t for the sandbox, which is when
+	 * hostfs interface is used
+	 */
+	return rbdd != NULL;
 }
 
 int sandbox_fs_read_at(const char *filename, loff_t pos, void *buffer,