makefiles: riotboot_serial: default to STDIO BAUD for PROG_BAUD
makefiles: riotboot_serial: default to STDIO BAUD for PROG_BAUD

If $(PROG_BAUD) is not set, default to the boards $(BAUD).

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