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The `toc_entry` and `image` data structures had a cyclic
relationship.  This patch removes the explicit dependencies and introduces
functions to link them via the UUID.

This change highlights the intent of the code better and makes it more
flexible for future enhancements.

Change-Id: I0c3dd7bfda2a631a3827c8ba4831849c500affe9
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>