Add support for printing version at runtime

Print out Trusted Firmware version at runtime at each BL stage.
Message consists of TF version as defined statically in the Makefile
(e.g. v0.4), build mode (debug|release) and a customizable build
string:

  1. By defining BUILD_STRING in command line when building TF
  2. Default string is git commit ID
  3. Empty if git meta-data is not available

Fixes ARM-software/tf-issues#203

Change-Id: I5c5ba438f66ab68810427d76b49c5b9177a957d6
Merge pull request #173 from jcastillo-arm/jc/tf-issues/203-v2

Add support for printing version at runtime