sandbox: enable poweroff command

The command to shut down a device is 'poweroff'. It is a deficit of the
sandbox that it does not support resetting yet but shuts down upong seeing
the 'reset' command.

Once the sandbox properly supports reset we need the 'poweroff' command to
leave the sandbox.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>