Move up to mbed TLS 2.x ...

The mbed TLS library has introduced some changes in the API from
the 1.3.x to the 2.x releases. Using the 2.x releases requires
some changes to the crypto and transport modules.

This patch updates both modules to the mbed TLS 2.x API.

All references to the mbed TLS library in the code or documentation
have been updated to 'mbed TLS'. Old references to PolarSSL have
been updated to 'mbed TLS'.

User guide updated to use mbed TLS 2.2.0.

NOTE: moving up to mbed TLS 2.x from 1.3.x is not backward compatible.
Applying this patch will require an mbed TLS 2.x release to be used.
Also note that the mbed TLS license changed to Apache version 2.0.

Change-Id: Iba4584408653cf153091f2ca2ee23bc9add7fda4

TBB: add authentication framework documentation ...

This patch updates the user guide, adding instructions to build the
Trusted Firmware with Trusted Board Support using the new framework.

It also provides documentation about the framework itself, including
a detailed section about the TBBR implementation using the framework.

Change-Id: I0849fce9c5294cd4f52981e7a8423007ac348ec6