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/drivers/mtd/spi/Kconfig b/drivers/mtd/spi/Kconfig
index 7b858a3..a9617c6 100644
--- a/drivers/mtd/spi/Kconfig
+++ b/drivers/mtd/spi/Kconfig
@@ -80,6 +80,14 @@
 
 if SPI_FLASH
 
+config BOOTDEV_SPI_FLASH
+	bool "SPI Flash bootdev support"
+	help
+	  Enable a boot device for SPI flash. This allows reading a script
+	  from SPI flash so that it can be used to boot an Operating System.
+
+	  If unsure, say N
+
 config SPI_FLASH_SFDP_SUPPORT
 	bool "SFDP table parsing support for SPI NOR flashes"
 	depends on !SPI_FLASH_BAR