sandbox: move source files from board/ to arch/sandbox/

Prior to commit 33a02da0, all boards must have board/${BOARD}/
or board/${VENDOR}/${BOARD}/ directory.
Now this rule is obsolete.

It looks weird that sandbox defines "vendor" and "board" just for
meeting the old U-Boot directory structure.

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