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Migrate all direct usage of __attribute__ to usage of their
corresponding macros from cdefs.h.
e.g.:
 - __attribute__((unused)) -> __unused
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>

This patch adds the authentication framework that will be used as
the base to implement Trusted Board Boot in the Trusted Firmware.
The framework comprises the following modules:

- Image Parser Module (IPM)

    This module is responsible for interpreting images, check
    their integrity and extract authentication information from
    them during Trusted Board Boot.

    The module currently supports three types of images i.e.
    raw binaries, X509v3 certificates and any type specific to
    a platform. An image parser library must be registered for
    each image type (the only exception is the raw image parser,
    which is included in the main module by default).

    Each parser library (if used) must export a structure in a
    specific linker section which contains function pointers to:

        1. Initialize the library
        2. Check the integrity of the image type supported by
           the library
        3. Extract authentication information from the image

- Cryptographic Module (CM)

    This module is responsible for verifying digital signatures
    and hashes. It relies on an external cryptographic library
    to perform the cryptographic operations.

    To register a cryptographic library, the library must use the
    REGISTER_CRYPTO_LIB macro, passing function pointers to:

        1. Initialize the library
        2. Verify a digital signature
        3. Verify a hash

    Failing to register a cryptographic library will generate
    a build time error.

- Authentication Module (AM)

    This module provides methods to authenticate an image, like
    hash comparison or digital signatures. It uses the image parser
    module to extract authentication parameters, the crypto module
    to perform cryptographic operations and the Chain of Trust to
    authenticate the images.

    The Chain of Trust (CoT) is a data structure that defines the
    dependencies between images and the authentication methods
    that must be followed to authenticate an image.

The Chain of Trust, when added, must provide a header file named
cot_def.h with the following definitions:

- COT_MAX_VERIFIED_PARAMS

    Integer value indicating the maximum number of authentication
    parameters an image can present. This value will be used by the
    authentication module to allocate the memory required to load
    the parameters in the image descriptor.

Change-Id: Ied11bd5cd410e1df8767a1df23bb720ce7e58178