z2: Make providing flash images non-mandatory

z2: Make providing flash images non-mandatory

Up to now, the z2 machine only boots if a flash image is provided.
This is not really necessary; the machine can boot from initrd or from
SD without it. At the same time, having to provide dummy flash images
is a nuisance and does not add any real value. Make it optional.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200217210903.18602-1-linux@roeck-us.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>