Fix overflows

Eventually, the stimulus 0 unshielded write would overflow, and once it
had, it stayed overflowed.  (At least as far as the traces show)
Commit updated version of libopencm3 Im using

THought this was already done :(
[swo] Update example to follow latest api
[swo] Add 32bit write on third channel.

Tidy up example comments and simply not even initialize the ITM
ourselves.  We let the debugger sw do this.