CMake: Add CMakeLists.txt

[rajkan01]

• Added CMakeLists.txt file to TLS examples so that it can build via CMake
• Added .travis.yml config to pick this example for Travis CI build
• Update mbed-os.lib to latest master to pick CMake changes

@0xc0170 @hugueskamba

[rajkan01]

@Patater Could you review this PR for adding CMake support to tls example