docs/devel/build-system: Update an example

The default-configs/ example added in 717171bd is no
more accurate since fa212a2b (and various further other
commits).

The Kconfig build system is now in place.
Use the aarch64-softmmu config as example.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190529140504.21580-1-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>