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

This reverts commit 25806090.

Conflicts:
	boards.cfg

Wrong patch 25806090 was applied by accident. Revert it.

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