net: websocket: Initial support for server websocket

This commit creates a websocket library that can be used by
applications. The websocket library implements currently only
server role and it uses services provided by net-app API.
The library supports TLS if enabled in configuration file.

This also adds websocket calls to HTTP app server if websocket
connection is established.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>