loader: More changes for gh181

loader: More changes for gh181

With Piers' fix, another path was broke because we still needed
to intercept the call before it went to the ICD.  Now, with his
change and this change, all paths work.

Also, clean up some code based on comments from Ian and Courtney
reviews.

Finally, update docs to indicate behavior of new loader 3
interface with ICDs.

Change-Id: I2d3f962baffb21c1edeb93b132ffad40f298c8e7