tls-client does not check that network stack and ethernet are available

The tls-client example does not check whether the networking features (e.g. ethernet) are available in the target device at compile-time. This can result in various failures at compile/run time that are not necessarily easy to debug. Preprocessor checks should be added to the example to ensure that a comprehensible error message is printed if ethernet is not available.

Start at the tls-client example entry point and inspect how networking features such as ethernet are detected or configured at compile time. Add checks so a target without ethernet produces a comprehensible error, then verify the example's behavior for unavailable networking features.

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