Add ECDH+KBKDF example

Add ECDH+KBKDF example

Add an ECDH+KBKDF example in form of a doxygen snippet. The code used in it is
compiled together with the project.

Disclaimer:
I don't know how to generate Tizen API documentation. Basing on other projects
I made an assumption that doxygen's EXAMPLE_PATH points to doc subdirectory.
With such assumption, vanilla doxygen generates the code snippet properly.

Change-Id: I7cdc98ff204238be0ad2440a8816ed53a61e7efc