Fix outdated SCons build scripts that were broken for WiFi.

Fix outdated SCons build scripts that were broken for WiFi.

Corrected outdated SCons build script that was still referencing the
'NET' environment variable instead of the TARGET_TRANSPORT one that
the CA branch was switched to using.

Removed setup/documentation of the outdated 'NET' option.

Change-Id: I2f22921b290da65d77fccb237b09dd0b6302996d
Signed-off-by: Jon A. Cruz <jonc@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/659
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Erich Keane <erich.keane@intel.com>