sandbox: Add architecture header files

This adds required header files for the sandbox architecture, and a basic
description of what sandbox is (README.sandbox).

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/common.h b/include/common.h
index 77a29c8..4c3e3a6 100644
--- a/include/common.h
+++ b/include/common.h
@@ -311,6 +311,9 @@
 #ifdef CONFIG_X86		/* x86 version to be fixed! */
 # include <asm/u-boot-x86.h>
 #endif /* CONFIG_X86 */
+#ifdef CONFIG_SANDBOX
+# include <asm/u-boot-sandbox.h>	/* TODO(sjg) what needs to be fixed? */
+#endif
 
 #ifdef CONFIG_AUTO_COMPLETE
 int env_complete(char *var, int maxv, char *cmdv[], int maxsz, char *buf);