Some machines don't provide _TZD, so check the availability of it
before carrying out futher operations.

If _TZD is present, also check the result of its evaluation.

[rjw: Changelog]
Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>