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The modified implementation uses the IO abstraction rather than
making direct semi-hosting calls.  The semi-hosting driver is now
registered for the FVP platform during initialisation of each boot
stage where it is used.  Additionally, the FVP platform includes a
straightforward implementation of 'plat_get_image_source' which
provides a generic means for the 'load_image' function to determine
how to access the image data.

Change-Id: Ia34457b471dbee990c7b3c79de7aee4ceea51aa6