sandbox: Move CONFIG_SPI_FLASH_SANDBOX to Kconfig

Move this over to Kconfig and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>

configs/sandbox_defconfig

@@ -21,3 +21,4 @@ CONFIG_SANDBOX_GPIO=y
 CONFIG_SYS_VSNPRINTF=y
 CONFIG_SYS_I2C_SANDBOX=y
 CONFIG_SANDBOX_SPI=y
+CONFIG_SPI_FLASH_SANDBOX=y

drivers/mtd/spi/Kconfig

@@ -12,3 +12,13 @@ config DM_SPI_FLASH
 	  during the transition parent. SPI and SPI flash must be
 	  enabled together (it is not possible to use driver model
 	  for one and not the other).
+
+config SPI_FLASH_SANDBOX
+	bool "Support sandbox SPI flash device"
+	depends on SANDBOX && DM_SPI_FLASH
+	help
+	  Since sandbox cannot access real devices, an emulation mechanism is
+	  provided instead. Drivers can be connected up to the sandbox SPI
+	  bus (see CONFIG_SANDBOX_SPI) and SPI traffic will be routed to this
+	  device. Typically the contents of the emulated SPI flash device is
+	  stored in a file on the host filesystem.

include/configs/sandbox.h

@@ -87,7 +87,6 @@
 #define CONFIG_SPI_FLASH_EON
 #define CONFIG_SPI_FLASH_GIGADEVICE
 #define CONFIG_SPI_FLASH_MACRONIX
-#define CONFIG_SPI_FLASH_SANDBOX
 #define CONFIG_SPI_FLASH_SPANSION
 #define CONFIG_SPI_FLASH_SST
 #define CONFIG_SPI_FLASH_STMICRO