sandbox: exynos: Move CONFIG_CMD_SOUND to Kconfig

Move this over to Kconfig and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/arndale_defconfig b/configs/arndale_defconfig
index 71c9e65..f452b13 100644
--- a/configs/arndale_defconfig
+++ b/configs/arndale_defconfig
@@ -4,3 +4,4 @@
 CONFIG_TARGET_ARNDALE=y
 CONFIG_DEFAULT_DEVICE_TREE="exynos5250-arndale"
 CONFIG_SOUND=y
+CONFIG_CMD_SOUND=y