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/include/bootflow.h b/include/bootflow.h
index 735ed87..319dda8 100644
--- a/include/bootflow.h
+++ b/include/bootflow.h
@@ -332,6 +332,15 @@
 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
  *
  * This checks the bootdev in the bootflow to make sure it uses a network