Tweak main build_common scripts

Tweak main build_common scripts

build_common/SConscript:
  * list of supported target-os not general enough for helpmsg, updated
  * send error msg to stderr by passing it to Exit instead of doing print
  * Use simpler python slicing in Yocto tool setup
  * pull most Add(somekind of construction variable) into one big call
    to AddVariables; name some more arguments for better clarity
  Removed temporarily while chasing Jenkins fails:
  * turn all variables which behave like booleans actually into BoolVariable
build_common/external_libs.scons:
  * a bad combination of options used to "raise" an error, but there is
    no need for a stack bracktrace here, want just a simple msg+exit
  * send error msg to stderr by passing it to Exit instead of doing print
build_common/external_libs.scons
  * send error msg to stderr by passing it to Exit instead of doing print
build_common/linux/SConscript
build_common/windows/SConscript
  * reduce repeated calls to env.get for same key
  * windows: update style to like in linux script

Bug: https://jira.iotivity.org/browse/IOT-1745
Change-Id: I928199f005af805cad0b0657ca62411caf7a2500
Signed-off-by: Mats Wichmann <mats@linux.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21311
Reviewed-by: Ibrahim Esmat <iesmat@microsoft.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: George Nash <george.nash@intel.com>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>