Auto generate platform ID.

Auto generate platform ID.

This allows all apps running on a platform (e.g. an installation of OS)
to register with the same platform ID in the platform info.

Applications can override this value by using the regular
OCSetPropertyValue(OC_RSRVD_PLATFORM_ID).

Change-Id: I638ab08a5e0a6319524052b95ac649f167e52a90
Signed-off-by: Soemin Tjong <stjong@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18103
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>