Let's assume that you are not change the OS itself but want to implement or extend platform-specific code.
In this case, there are six, maybe seven, places where you can provide the platform code.
http://nuttx.org/Documentation/directories.png?800x391
REVISIT: This is a out of date. There has been a recent reorganization of the board-related directories: The configs/ directory has been renamed boards/ and structure under the new boards/ directory is significantly different. A <board> directory would now be found at boards/<arch>/<chip>/<board>. Conceptually the figure is correct, just not consistent with current naming.
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