Code cleanup of csdk/connectivy/util/SConscript

Code cleanup of csdk/connectivy/util/SConscript

* Base directory paths are more clear than relative paths.
* Remove unnecessary reassignment of src_dir.

Apply change to extlibs/tinydtls/SConscript to reduce env pollution.
This includes a fix for a newly exposed issue wherein debug.h is being
included from an incorrect directory.

Change-Id: Icd6a8afe688768da8e491006f2479cbbe78ca02a
Signed-off-by: David Antler <david.a.antler@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/5525
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jon A. Cruz <jon@joncruz.org>