When hw/$DIR/Kconfig is changed, the corresponding generated
hw/$DIR/config-devices.mak is not being updated.
Fix this by including all the hw/*/Kconfig files to the prerequisite
names of the rule generating the config-devices.mak files.

Fixes: e0e312f3 (build: switch to Kconfig)
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>