Ensure we're using the right contiki upstream
Ensure LSE is turned on before we try and use it.

Earlier code only worked because I'd configured the RTC earlier, and
hadn't power cycled the board :|
Eliminate warnings from dead/unused code
WIP rtimer impl.

It triggers, but not always at the intervals I expect, and it's not
interrupt safe.  Calling print in the rtimer task explodes things.  (And
that's "correct" right?