spl: fix entry_point equal to load_addr

At the moment entry_point is set to image_get_load(header) that sets it
to "load address" instead of "entry point", assuming entry_point is
equal to load_addr, but it's not true. Then load_addr is set to
"entry_point - header_size", but this is wrong too since load_addr is
not an entry point.

So use image_get_ep() for entry_point assignment and image_get_load()
for load_addr assignment.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>