TBB: add TBBR Chain of Trust ...

This patch adds a CoT based on the Trusted Board Boot Requirements
document*. The CoT consists of an array of authentication image
descriptors indexed by the image identifiers.

A new header file with TBBR image identifiers has been added.
Platforms that use the TBBR (i.e. ARM platforms) may reuse these
definitions as part of their platform porting.

PLATFORM PORT - IMPORTANT:

Default image IDs have been removed from the platform common
definitions file (common_def.h). As a consequence, platforms that
used those common definitons must now either include the IDs
provided by the TBBR header file or define their own IDs.

*The NVCounter authentication method has not been implemented yet.

Change-Id: I7c4d591863ef53bb0cd4ce6c52a60b06fa0102d5