sandbox: Move CONFIG_SPI_FLASH_SANDBOX to Kconfig

Move this over to Kconfig and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 058949a..dd33028 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -21,3 +21,4 @@
 CONFIG_SYS_VSNPRINTF=y
 CONFIG_SYS_I2C_SANDBOX=y
 CONFIG_SANDBOX_SPI=y
+CONFIG_SPI_FLASH_SANDBOX=y