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>

include/bootflow.h

@@ -331,6 +331,15 @@ void bootflow_remove(struct bootflow *bflow);
  */
 int bootflow_iter_check_blk(const struct bootflow_iter *iter);
 
+/**
+ * bootflow_iter_check_sf() - Check that a bootflow uses SPI FLASH
+ *
+ * This checks the bootdev in the bootflow to make sure it uses SPI flash
+ *
+ * Return: 0 if OK, -ENOTSUPP if some other device is used (e.g. ethernet)
+ */
+int bootflow_iter_check_sf(const struct bootflow_iter *iter);
+
 /**
  * bootflow_iter_check_net() - Check that a bootflow uses a network device
  *