sandbox: Add basic SPL implementation

Add an sandbox implementation for the generic SPL framework. This supports
locating and running U-Boot proper.

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

arch/sandbox/include/asm/spl.h

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2016 Google, Inc
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#ifndef __asm_spl_h
+#define __asm_spl_h
+
+#define CONFIG_SPL_BOARD_LOAD_IMAGE
+
+/**
+ * Board-specific load method for boards that have a special way of loading
+ * U-Boot, which does not fit with the existing SPL code.
+ *
+ * @return 0 on success, negative errno value on failure.
+ */
+int spl_board_load_image(void);
+
+enum {
+	BOOT_DEVICE_BOARD,
+};
+
+#endif