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

[andresag01]

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.

[ciarmcom]

ARM Internal Ref: IOTSSL-1183