NuttX merge / update

*DO NOT MERGE YET*

This branch is not ready for merging yet, but to raise that a NuttX update is pending and getting into into shape. I want to do one more NuttX update before proposing wider testing.

Some assorted comments:
  * Testing showed fine operation, but Anton's custom NSH extensions have been replaced with Greg Nutt's version of it. This will need an update of the defconfig to enable it and some more testing.
  * Rest of devices and sensors seem to behave as expected.