To allow operating on images with unknown UUIDs, fiptool needs to
be able to track an arbitrary amount of images and not be limited
to the set of images described by the builtin table.

Convert the table to a list to accommodate this scenario.

Change-Id: I0e6d738eece7795d74fc72d165a3098f223d4414
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>