doc: detail methods for configuring Refkit images.
doc: detail methods for configuring Refkit images.

v2: update the original text (in PR #217) with review
suggestions from Patrick Ohly.

Signed-off-by: Ismo Puustinen <75dda586a9213f0e0695eb79120c94222bb30e60@intel.com>

mythi/intel-iot-refkit,klihub/intel-iot-refkit,klihub/intel-iot-refkit,YinThong/intel-iot-refkit,jairglez/intel-iot-refkit,klihub/intel-iot-refkit,intel/intel-iot-refkit,YinThong/intel-iot-refkit,ipuustin/intel-iot-refkit,mythi/intel-iot-refkit,ipuustin/intel-iot-refkit,YinThong/intel-iot-refkit,mythi/intel-iot-refkit,intel/intel-iot-refkit,mythi/intel-iot-refkit,ipuustin/intel-iot-refkit,intel/intel-iot-refkit,intel/intel-iot-refkit,mythi/intel-iot-refkit,mythi/intel-iot-refkit,jairglez/intel-iot-refkit,jairglez/intel-iot-refkit,klihub/intel-iot-refkit,klihub/intel-iot-refkit,mythi/intel-iot-refkit,jairglez/intel-iot-refkit,ipuustin/intel-iot-refkit,klihub/intel-iot-refkit,ipuustin/intel-iot-refkit,YinThong/intel-iot-refkit,jairglez/intel-iot-refkit,intel/intel-iot-refkit,YinThong/intel-iot-refkit,YinThong/intel-iot-refkit,YinThong/intel-iot-refkit,jairglez/intel-iot-refkit,intel/intel-iot-refkit,ipuustin/intel-iot-refkit,klihub/intel-iot-refkit,intel/intel-iot-refkit,jairglez/intel-iot-refkit,ipuustin/intel-iot-refkit