Build: Follow the 'source_structure.rst'

This is the first patch to follow the first commit of source structure
document. The items under the 'secure_fw' folder are re-organized:

- Create/Move some folders/files to follow document.
- Rename some folders to foll, for example, 'secure_fw/services' to
  'secure_fw/partitions'.
- Update affected files to make it work.

This is a big change, to make the structure meet the basic shape of
the structure document defined, and make it easier to be understood
for users. Staging changes are not applicable so they are combined
into one - and because it is not the final shape yet, so:

- Upcoming updates on the 'secure_fw' folder would follow up soon.
- Fine-tune about the 'source_structure.rst' would come, too.

Change-Id: I5c11175e0a4579cd9b42d3e3519dbffb87334d0b
Signed-off-by: Ken Liu <ken.liu@arm.com>