boards/mcb2388: define flank of the on-board button
boards/mcb2388: define flank of the on-board button

Define the button flank for tests that use this define.

OlegHahm/RIOT,authmillenon/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,miri64/RIOT,basilfx/RIOT,OTAkeys/RIOT,jasonatran/RIOT,ant9000/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,miri64/RIOT,miri64/RIOT,miri64/RIOT,ant9000/RIOT,kYc0o/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,kYc0o/RIOT,basilfx/RIOT,jasonatran/RIOT,authmillenon/RIOT,ant9000/RIOT,kYc0o/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,basilfx/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,jasonatran/RIOT,basilfx/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,ant9000/RIOT,basilfx/RIOT,kaspar030/RIOT,authmillenon/RIOT,kaspar030/RIOT,authmillenon/RIOT