Add smm-variable standalone service context

To support service-level testing of the smm-variable service in
a native PC environment, a standalone service context has been
added that allows the smm-variable service to be discovered
and used via the service locator API exposed by libts.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: I1998c0a80f325938498f5edf7b36abd170e06cc8
6 files changed