[cmake] add option to enable secure settings API (#7517)
[cmake] add option to enable secure settings API (#7517)

This commit adds an option to the POSIX platform CMake module to turn
on/off the secure settings API.
openthread/openthread,jwhui/openthread,srickardti/openthread,srickardti/openthread,openthread/openthread,jwhui/openthread,jwhui/openthread,abtink/openthread,openthread/openthread,abtink/openthread,jwhui/openthread,srickardti/openthread,openthread/openthread,abtink/openthread,abtink/openthread,srickardti/openthread