platform/mulle: Clean up udelay implementation.

Use F_BUS to compute the right timer value when waiting.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60/onewire: 1-wire reset working.

Switchable mode, two wire mode is more reliable if many interrupts
are coming in at the same time but will require two pins on the CPU
and a jumper between them.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>