sandbox: set sandbox's vendor to null

Because sandbox is not a real hardware, setting vendor=sandbox is
almost meaningless.

This commit sets sandbox's vendor field to '-'.

It is a good thing that it decreases one level directory hierarchy.
The files board/sandbox/sandbox/* have been moved to board/sandbox/*.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>