Doc: Add architectural overview of the operating system.

The architectural overview of the OS contains the general information
regarding the nanokernel, the microkernel, the driver model and the
application model.

Change-Id: I5a16f3f5ec5023239a772d3e559bd761795c27d4
Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>