refactor(memmap): create common image parser interface

This is another non-functional change, which factors the memory
footprint and symbol listing interfaces shared by the two image parsing
backends into a common `Image` interface.

Change-Id: I625a8e5ca77e38155b6ed545a33a8ae4c2cf133a
Signed-off-by: Chris Kay <chris.kay@arm.com>
4 files changed