IOT-1583: Removing /W4 warnings from mbedtls.

IOT-1583: Removing /W4 warnings from mbedtls.

Removing all /W4 warnings from mbedtls and adding
/WX to the mbedtls' SConscript to make sure no new ones
get added.

I'm adding a separate header for Windows builds, because
they require a different set of macros to build correctly
("MBEDTLS_HAVE_WINSOCK2" has to be available). The config
headers are not generated automatically for mbedTLS and
a default version is just checked into the repo, so each
separate configuration/platform build requires a separate
header.

Change-Id: I3b423fb2d0b978da5e9907e58df6b6818533ba22
Signed-off-by: Pawel Winogrodzki <pawelwi@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18013
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>
Reviewed-by: Kevin Kane <kkane@microsoft.com>