sandbox: Add a new sandbox_spl board

It is useful to be able to build SPL for sandbox. It provides additional
build coverage and allows SPL features to be tested in sandbox. However
it does not need worthwhile to always create an SPL build. It nearly
doubles the build time and the feature is (so far) seldom used.

So for now, create a separate build target for sandbox SPL. This allows
experimentation with this new feature without impacting existing workflows.

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

include/configs/sandbox_spl.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2016 Google, Inc
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#ifndef __SANDBOX_SPL_CONFIG_H
+#define __SANDBOX_SPL_CONFIG_H
+
+#include <configs/sandbox.h>
+
+#define CONFIG_SPL_DRIVERS_MISC_SUPPORT
+#define CONFIG_SPL_ENV_SUPPORT
+#define CONFIG_SPL_FRAMEWORK
+#define CONFIG_SPL_LIBCOMMON_SUPPORT
+#define CONFIG_SPL_LIBGENERIC_SUPPORT
+#define CONFIG_SPL_SERIAL_SUPPORT
+
+#endif