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/Makefile b/drivers/mtd/spi/Makefile
index 99cc418..4093953 100644
--- a/drivers/mtd/spi/Makefile
+++ b/drivers/mtd/spi/Makefile
@@ -21,3 +21,4 @@
 obj-$(CONFIG_SPI_FLASH_DATAFLASH) += sf_dataflash.o
 obj-$(CONFIG_$(SPL_TPL_)SPI_FLASH_MTD) += sf_mtd.o
 obj-$(CONFIG_SPI_FLASH_SANDBOX) += sandbox.o
+obj-$(CONFIG_$(SPL_TPL_)BOOTDEV_SPI_FLASH) += sf_bootdev.o