Simplifies mbedtls CMakeLists.txt

[shikokuchuo]

Fixes #1771 The new findMbedTLS from NNG 1.7.2 fails

The first commit simplifies the logic such that is works again (as described in the issue).
The second commit just tidies up the new BSD socket transport documentation.

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[gdamore]

I see one typo in the original code. I am not sure that I want to take this entire change. It may be best just to fix the typo.

[shikokuchuo]

Correcting the typo didn't change anything when I tested. I've updated the issue - something is still not quite right it seems.

[gdamore]

This change is wrong. And I've tested this reasonably myself with a variety of settings. I think something else is wrong in your environment. I'm closing this PR for now, because I'm not going to accept it as is. I think more investigation of the problem is required first.