coding standard: Add a .editorconfig based on the Nuttx code standards

This file provide reference settings for the Nuttx code conventions
There are plug-ins (with source) available for nearly every text editor
to automatically respect the conventions contained within this file.

Please see editorconfig.org for complete information on how to use this file.
For the Nuttx coding standard see: http://nuttx.org/doku.php?id=documentation:codingstandard