greybus: tools: don't always build them.

This breaks the kernel-only build as it can't find any userspace headers
with the cross-compiler, so don't build the tools by "default" unless
you ask for them.

Reported-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>