boards/cc2538dk: Changed UART config format
boards/cc2538dk: Changed UART config format

Changed the configuration of the cc2538dk board to fit the new
cc2538 format defined in the previous commit. That is, from a defines
based format to a struct based format.

RIOT-OS/RIOT,josephnoir/RIOT,cladmi/RIOT,rfuentess/RIOT,OlegHahm/RIOT,biboc/RIOT,A-Paul/RIOT,mfrey/RIOT,lazytech-org/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,mtausig/RIOT,basilfx/RIOT,smlng/RIOT,cladmi/RIOT,x3ro/RIOT,lazytech-org/RIOT,josephnoir/RIOT,biboc/RIOT,aeneby/RIOT,jasonatran/RIOT,OTAkeys/RIOT,ant9000/RIOT,x3ro/RIOT,A-Paul/RIOT,gebart/RIOT,gebart/RIOT,lazytech-org/RIOT,A-Paul/RIOT,miri64/RIOT,authmillenon/RIOT,authmillenon/RIOT,kYc0o/RIOT,miri64/RIOT,toonst/RIOT,aeneby/RIOT,gebart/RIOT,x3ro/RIOT,avmelnikoff/RIOT,jasonatran/RIOT,avmelnikoff/RIOT,rfuentess/RIOT,x3ro/RIOT,avmelnikoff/RIOT,smlng/RIOT,smlng/RIOT,kYc0o/RIOT,josephnoir/RIOT,authmillenon/RIOT,gebart/RIOT,yogo1212/RIOT,avmelnikoff/RIOT,biboc/RIOT,aeneby/RIOT,OTAkeys/RIOT,toonst/RIOT,kbumsik/RIOT,ant9000/RIOT,rfuentess/RIOT,authmillenon/RIOT,OlegHahm/RIOT,BytesGalore/RIOT,cladmi/RIOT,kbumsik/RIOT,avmelnikoff/RIOT,A-Paul/RIOT,gebart/RIOT,authmillenon/RIOT,yogo1212/RIOT,josephnoir/RIOT,kaspar030/RIOT,x3ro/RIOT,RIOT-OS/RIOT,mtausig/RIOT,toonst/RIOT,toonst/RIOT,mfrey/RIOT,kaspar030/RIOT,smlng/RIOT,smlng/RIOT,miri64/RIOT,OTAkeys/RIOT,rfuentess/RIOT,RIOT-OS/RIOT,aeneby/RIOT,biboc/RIOT,kaspar030/RIOT,ant9000/RIOT,BytesGalore/RIOT,yogo1212/RIOT,neiljay/RIOT,rfuentess/RIOT,aeneby/RIOT,cladmi/RIOT,BytesGalore/RIOT,neiljay/RIOT,mfrey/RIOT,yogo1212/RIOT,BytesGalore/RIOT,toonst/RIOT,miri64/RIOT,kYc0o/RIOT,mfrey/RIOT,mtausig/RIOT,A-Paul/RIOT,neiljay/RIOT,ant9000/RIOT,neiljay/RIOT,kbumsik/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,basilfx/RIOT,biboc/RIOT,cladmi/RIOT,jasonatran/RIOT,BytesGalore/RIOT,kaspar030/RIOT,lazytech-org/RIOT,yogo1212/RIOT,kYc0o/RIOT,kYc0o/RIOT,mtausig/RIOT,neiljay/RIOT,jasonatran/RIOT,basilfx/RIOT,RIOT-OS/RIOT,mtausig/RIOT,miri64/RIOT,authmillenon/RIOT,kaspar030/RIOT,basilfx/RIOT,OlegHahm/RIOT,kbumsik/RIOT,lazytech-org/RIOT,josephnoir/RIOT,OTAkeys/RIOT,mfrey/RIOT,ant9000/RIOT,jasonatran/RIOT,kbumsik/RIOT,basilfx/RIOT,OlegHahm/RIOT