Update scons build readme and howto document

Update scons build readme and howto document

Change-Id: I3a9e7dd82ad98f969449da42acadfad2e0eb4705
Signed-off-by: Caiwen Zhang<caiwen.zhang@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/432
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Patrick Lankswert <patrick.lankswert@intel.com>
(cherry picked from commit f7c465f6d306179b4244591cfe0be73a14feb047)
Reviewed-on: https://gerrit.iotivity.org/gerrit/472
Reviewed-by: Sudarshan Prasad <sudarshan.prasad@intel.com>