IOT-1583: Preparing connectivity scons script for /W3 changes (part 1).

IOT-1583: Preparing connectivity scons script for /W3 changes (part 1).

First batch of changes connectivity scons scripts to allow
enabling of /W3 level warnings for the connectivity directory
only.

This change makes sure the connectivity Environment variable
is separated from the global "env", so it only modifies the way
connectivity is built.

Change-Id: I76e9f742bb462b8b9dd6c5c26e07d9c553d70446
Signed-off-by: Pawel Winogrodzki <pawelwi@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/17367
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: Ashok Babu Channa <ashok.channa@samsung.com>