spl: Support selecting images based on phase in simple FIT

spl: Support selecting images based on phase in simple FIT

At present the simple FIT-loader always loads images, ignoring whether
they are intended for the next phase or not.

VBE packages up several images in the same FIT, some destined for VPL
and some for SPL. Add logic to check the phase before loading the
image. Return -EPERM in that case and handle it gracefully.

Fix a unnecessary re-computation of read_offset while here.

Signed-off-by: Simon Glass <sjg@chromium.org>