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/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 299db4b..5de7fbe 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -25,3 +25,4 @@
 CONFIG_TPM_TIS_SANDBOX=y
 CONFIG_SOUND=y
 CONFIG_CMD_SOUND=y
+CONFIG_SOUND_SANDBOX=y