sandbox: Finish migration to Kconfig

Stop using CONFIG_SANDBOX_ARCH and use CONFIG_SANDBOX instead. For the
SPI related defines, set them directly in Kconfig. This now empties
arch/sandbox/include/asm/config.h.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>