IOT-1583: Disabling rebuild of config headers with every scons run.

IOT-1583: Disabling rebuild of config headers with every scons run.

I'm changing the scons behavior to only rebuild the config headers
if they don't exist, so scons wouldn't rebuild everything, what depends
on these headers.

I'm also making small fixes to make sure the generated headers are removed
when scons is run with "-c".

Change-Id: I4b8cf9cafac26e8d619280dccb1c14dab7c437cc
Signed-off-by: Pawel Winogrodzki <pawelwi@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/17415
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>