Added checks to FWU_SMC_IMAGE_COPY to prevent loading data into a
memory region where another image data is already loaded.

Without this check, if two images are configured to be loaded in
overlapping memory regions, one of them can be loaded and
authenticated and the copy function is still able to load data from
the second image on top of the first one. Since the first image is
still in authenticated state, it can be executed, which could lead to
the execution of unauthenticated arbitrary code of the second image.

Firmware update documentation updated.

Change-Id: Ib6871e569794c8e610a5ea59fe162ff5dcec526c
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>