sandbox: Allow SPI flash bootdevs to be disabled for tests

Most tests don't want these and they can create a lot of noise. Add a way
to disable them. Use that in tests, with a flag provided to enable them
for tests that need this feature.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/include/asm/state.h b/arch/sandbox/include/asm/state.h
index f125fb8..59a2059 100644
--- a/arch/sandbox/include/asm/state.h
+++ b/arch/sandbox/include/asm/state.h
@@ -97,6 +97,7 @@
 	bool handle_signals;		/* Handle signals within sandbox */
 	bool autoboot_keyed;		/* Use keyed-autoboot feature */
 	bool disable_eth;		/* Disable Ethernet devices */
+	bool disable_sf_bootdevs;	/* Don't bind SPI flash bootdevs */
 
 	/* Pointer to information for each SPI bus/cs */
 	struct sandbox_spi_info spi[CONFIG_SANDBOX_SPI_MAX_BUS]