bootstd: BOOTDEV_SPI_FLASH requires BOOTSTD

Compiling sandbox_defconfig with CONFIG_BOOTSTD=n fails:

    /usr/bin/ld: drivers/mtd/spi/sf_bootdev.o:
    in function `sf_get_bootflow':
    /drivers/mtd/spi/sf_bootdev.c:43:(.text+0x96):
    undefined reference to `bootmeth_set_bootflow'

Add the missing Kconfig dependency.

Fixes: Fixes: 0c1f4a9f ("bootstd: Add a SPI flash bootdev")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>