azure: Use --board flag with sandbox_spl

At present there is only one board which uses sandbox SPL. But with
sandbox_noinst being added, this is no longer true. Add a --board flag
so that we just build one board on azure, as is done in gitlab.

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