This patch adds support to not reserve the memory where an image is
loaded if the image is:

  1. A non-executable image e.g. a certificate
  2. An executable image which is not meant to run on the
     application CPU (e.g. BL3-0)

Both types of images are characterized by a NULL entrypoint argument
to the load_image() function. It is used to distinguish them from
other type of images.

Important: Use this feature carefully. The caller is responsible for
providing a valid entrypoint while loading images which will execute
on the application CPU to prevent a potential overwrite of the
corresponding memory region.

Change-Id: Ied482280d9db714c529ec12c33a6c1d918d77a4e