https://github.com/awslabs/aws-c-iot is not SDK, it's just used as one of the dependencies for C++ IoT SDK and provides some functionality related to IoT Device.

So, answering to your question. If you need C SDK to interact with AWS IoT services, https://github.com/aws/aws-iot-device-sdk-embedded-C looks like the right choice.

As for Yocto recipe. I found some tutorials and third-party recipes, but they're pretty outdated. I believe you'll need to make your own version.