Make UUID buffer optional for is_trusted_os_present()

The caller might simply want to know whether there is a Trusted OS,
without the need to identify it.

Change-Id: I97eef8b6e6c4cb948d48735cd7170fced98aee9a
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
2 files changed