Fix issues in FWU code

This patch fixes the following issues in Firmware Update (FWU) code:

1. The FWU layer maintains a list of loaded image ids and
   while checking for image overlaps, INVALID_IMAGE_IDs were not
   skipped. The patch now adds code to skip INVALID_IMAGE_IDs.

2. While resetting the state corresponding to an image, the code
   now resets the memory used by the image only if the image were
   copied previously via IMAGE_COPY smc. This prevents the invalid
   zeroing of image memory which are not copied but are directly
   authenticated via IMAGE_AUTH smc.

Change-Id: Idf18e69bcba7259411c88807bd0347d59d9afb8f
Signed-off-by: Soby Mathew <soby.mathew@arm.com>