bl2: post process only the loaded image

If an image was not loaded due to its SKIP_LOAD attribute, there is
no reason to post process the image. This change ensures only loaded
images get post processed.

Change-Id: Ib9f84f0f1dd7e811ec400fa999c84a15dd9b6297
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>