doc: declare CONFIG_XYZ with `.. option::` vs a header

This allows us to cross ref from code with :option:`CONFIG_XYZ`
generating a useful link and avoiding an 'undefined target` warning.

Hyperlink using :option: instead of :ref:.

Change-Id: I6cc0daec012dfcca504faa47d591885e69c8e521
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>