Make local functions and objects static

[casaroli]

Description

In NuttX flat build, all objects are linked together, so that they end up in a flat namespace.

If ssl_server2 and ssl_client2 are linked together, we get some multiple definitions of these functions.

It should not hurt the current use cases if set these to static.

Please accept this PR as part of a (hopefully short) series to make mbedtls more cleanly integrated in NuttX.

PR checklist

Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")

• [x] changelog provided, or not required
• [x] 3.6 backport done, or not required
• [x] 2.28 backport done, or not required
• [x] tests provided, or not required

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[misch7]

Hey @casaroli,

just yesterday I pushed some commits to the PR #9189, walking over the same code files and making a lot of stuff static. Could you please checkout and build the code and see if this fixes your issues as well?