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/sandbox_defconfig b/configs/sandbox_defconfig
index db10c19..299db4b 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -24,3 +24,4 @@
 CONFIG_SPI_FLASH_SANDBOX=y
 CONFIG_TPM_TIS_SANDBOX=y
 CONFIG_SOUND=y
+CONFIG_CMD_SOUND=y