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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

This patch adds the following mbedTLS based libraries:

* Cryptographic library

It is used by the crypto module to verify a digital signature
and a hash. This library relies on mbedTLS to perform the
cryptographic operations. mbedTLS sources must be obtained
separately.

Two key algorithms are currently supported:

    * RSA-2048
    * ECDSA-SECP256R1

The platform is responsible for picking up the required
algorithm by defining the 'MBEDTLS_KEY_ALG' variable in the
platform makefile. Available options are:

    * 'rsa' (for RSA-2048) (default option)
    * 'ecdsa' (for ECDSA-SECP256R1)

Hash algorithm currently supported is SHA-256.

* Image parser library

Used by the image parser module to extract the authentication
parameters stored in X509v3 certificates.

Change-Id: I597c4be3d29287f2f18b82846973afc142ee0bf0