Minor refactoring of BL1 FWU code

Minor refactoring of BL1 FWU code

This patch introduces no functional change, it just changes
the serial console output.

 - Improve accuracy of error messages by decoupling some
   error cases;

 - Improve comments;

 - Move declaration of 'mem_layout' local variable closer to
   where it is used and make it const;

 - Rename a local variable to clarify whether it is a source
   or a destination address (base_addr -> dest_addr).

Change-Id: I349fcf053e233f316310892211d49e35ef2c39d9
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Signed-off-by: Dan Handley <dan.handley@arm.com>