doc: fix :option: x-refs to squash doc generation warnings

Move to use :option:`CONFIG_XYZ` vs :option:`XYZ` to generate propert
links and avoid warnings about unexistant targets.

Change-Id: I4b46041f25e538462b123ccc8337f733033cc0e7
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>