sandbox: exynos: Move CONFIG_SOUND_SANDBOX to Kconfig

Move this over to Kconfig and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/sound/Kconfig b/drivers/sound/Kconfig
index a1c950b..3b96e84 100644
--- a/drivers/sound/Kconfig
+++ b/drivers/sound/Kconfig
@@ -37,6 +37,15 @@
 	  audio data and I2C for codec control. At present it only works
 	  with the Samsung I2S driver.
 
+config SOUND_SANDBOX
+	bool "Support sandbox emulated audio codec"
+	depends on SANDBOX && SOUND
+	help
+	  U-Boot sandbox can emulate a sound device using SDL, playing the
+	  sound on the host machine. This option implements the sound_init()
+	  and sound_play() functions for sandbox. Note that you must install
+	  the SDL libraries for this to work.
+
 config SOUND_WM8994
 	bool "Support Wolfson Micro wm8994 audio codec"
 	depends on I2S_SAMSUNG