sandbox: spi: Enable new spi/sf layers

We want to test SPI flash code in the sandbox, so enable the new drivers and
the 'sf test' command.

This command is used to validate the sandbox SPI / SPI flash implementation,
so enable it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Simon Glass <sjg@chromium.org>