coap-http-proxy: Removing redundant configuration

coap-http-proxy: Removing redundant configuration

Moving global environment configuration common for all
platforms into build_common/SConscript and removing repeated
lines from service/coap-http-proxy/*.

Bug: https://jira.iotivity.org/browse/IOT-1583
Change-Id: I6e88ec4553850340044a9788bf0c8782bc4283ad
Signed-off-by: Pawel Winogrodzki <pawelwi@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21323
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: Abhishek Sharma <ce.abhishek@samsung.com>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Tested-by: Phil Coval <philippe.coval@osg.samsung.com>