Make the entry point argument optional in load_image()

There are cases where the entry point information is useless to the
caller, e.g. when an image just needs to be loaded in memory but won't
ever be executed.

This patch allows load_image() function to take a NULL pointer as the
entry point argument. In this case, it won't be populated.

Change-Id: Ie9394b054457706c6699926c5e0206e0c3851c56
Merge pull request #128 from sandrine-bailleux/sb/make-load_image-ep-optional

Make the entry point argument optional in load_image()