iotile build - support for generating build_info.h compile information

Goal Statement via Tim:

> - Add the ability to `iotile-build` to generate another `.h` file during the build named something like `build_info.h`. This should pull in the version information from the `module_settings.json` including the prerelease details if any (such as type alpha, beta and prerelease number). It should also include a `#define` for the compilation timestamp and we'll likely shove some more things into it in the future.
> - The machinery is all there already so it should just be a quick modification to `arm.py` under `site_scons` to add another build action and then reference it from `build_library` and `build_program` in that same file.
> - The contents of the `module_settings.json` file should already be available as an `IOTile` object from the `tile` parameter to `build_library` and `build_program`