bootstd: Add a SPI flash bootdev

Add a bootdev for SPI flash so that these devices can be used with
standard boot. It only supports loading a script.

Add a special case for the label, since we want to use "spi", not
"spi_flash".

Enable the new bootdev on sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/bootflow.c b/boot/bootflow.c
index 52cc2f9..d2dbc9d 100644
--- a/boot/bootflow.c
+++ b/boot/bootflow.c
@@ -440,6 +440,18 @@
 	return -ENOTSUPP;
 }
 
+int bootflow_iter_check_sf(const struct bootflow_iter *iter)
+{
+	const struct udevice *media = dev_get_parent(iter->dev);
+	enum uclass_id id = device_get_uclass_id(media);
+
+	log_debug("uclass %d: %s\n", id, uclass_get_name(id));
+	if (id == UCLASS_SPI_FLASH)
+		return 0;
+
+	return -ENOTSUPP;
+}
+
 int bootflow_iter_check_net(const struct bootflow_iter *iter)
 {
 	const struct udevice *media = dev_get_parent(iter->dev);