working maca_on and maca_off
This unifies the rtimers and the etimers to both use the RTC. The
typicial contiki clock routine and etimer managment is performed as a
regularly scheduled rtimer.

This will simplify low-power operation as all timer based events will
be using the same timer that controls wake up and runs while sleeping.
add rtimer_arch_sleep and M12 pin config for low-power operations