Conflict with function name and variable name within that function.
Change the name of the function from image_size to get_image_size
to remove conflict and make the function fit the normal project
naming convention.

Rule 5.3:  An identifier declared in an inner scope shall not
           hide an identifier declared in an outer scope

Fixed For:
    make LOG_LEVEL=50 PLAT=fvp

Change-Id: I1a63d2730113e2741fffa79730459c584b0224d7
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>