GitLab

The user guide documentation for the cert_create tool needs to
mention that a platform must have a platform_oid.h header file
in order to successfully build the cert_create tool when
USE_TBBR_DEFS is 0.

Change-Id: I77f86a022d207e88a79c97741be3eafbfa0c86f1
Signed-off-by: Paul Beesley <paul.beesley@arm.com>