cpu/arm/k60: Move 1-wire configuration to platform config-board.h

Proper fix for gebart/contiki@9903404dff28d4cddd10c966837a44ad61fdcc20

 - Use UART0 by default (AH T1.8 Wheel Loader uses this port)
 - Added ONEWIRE_* macros to platform/mulle/config-board.h
 - Make ow_init use port_module_enable() instead of direct register
   manipulation.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>