sys/riotboot/flashwrite.c: add message to static assert
sys/riotboot/flashwrite.c: add message to static assert

static_assert as defined in c11 requires both an expression and a message.
The message is only optional from c17 onwards.

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