Allow FetchContent usage in external projects

[nicovanduijn]

Description

My project has an existing FreeRTOS Kernel and I attempted to include this project via CMake's FetchContent function as suggested by the Readme. In this process I stumbled over two minor details:

• Copy-paste error in the readme, suggesting to set defines for the FAT build instead of the TCP library
• No functionality to prevent the TCP library to pull in its own copy of the kernel, as other libraries have

Test Steps

• Set up a project with bare-bones FreeRTOS
• Consume this library via CMake's FetchContent as suggested by the readme

Checklist:

• [~] I have tested my changes. No regression in existing tests. -> I have obviously tested that this solution fixes my problem, but I'm unsure how I can test it against regressions in the upstream CI other than just opening this PR.
• [ ] I have modified and/or added unit-tests to cover the code changes in this Pull Request. -> No real code changes outside the build system, so this does not apply

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[kstribrnAmzn]

Thank you for submitting this PR! I took a look over it and saw nothing wrong. I'll send this to the TCP experts on the team for their review.