sandbox: Update minor documentation changes

- Use _defconfig instead of _config, but still _config is working.
- Corrected README.sandbox path in ./README

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index 0a0f528..276869a 100644
--- a/README
+++ b/README
@@ -272,7 +272,7 @@
 specific to be undertaken on a native platform. The sandbox is also used to
 run some of U-Boot's tests.
 
-See board/sandbox/sandbox/README.sandbox for more details.
+See board/sandbox/README.sandbox for more details.
 
 
 Configuration Options: