Mulle clock and rtimer refactoring

This PR changes the clock implementation to use the rtimer API, and the rtimer to use the LPTMR module instead of the PIT module.
The commits from contiki-os/contiki#495 are a prerequisite for allowing the rtimer to schedule more than one timer. This is necessary in order to use the rtimer for anything else while we are also using it for the clock ticks.