Add platform support

TS project structure and build system extended to accommodate
hardware specific drivers. The concept of a platform is introduced
to allow hardware specific drivers from external providers to be
used. This change implements the Portability Model described in
the project documentation.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: I61e678ae103e0bf139f2c440ba6cd010620af37e
24 files changed