ssl: enable usage of patched mbed functions conditionally

ssl: enable usage of patched mbed functions conditionally

Only activate support for the MBEDTLS_KEY_EXCHANGE_ECDH_ANON_ENABLED
cipher suite when this is actually available in mbedtls.
This makes it possible to compile IoTivity against a unpatched mbedtls
version.

Change-Id: Ic56d7faf7597cc5dc08f9d55be29dde898949609
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-on: https://gerrit.iotivity.org/gerrit/15121
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Reviewed-by: Greg Zaverucha <gregz@microsoft.com>