cert_create: Remove references to TBBR in common code

In preparation of supporting alternate chains of trust, reword comments
and error messages that explicitly mentioned TBBR.

Change-Id: I85a0b08e16d0cd82f3b767fcc092d1f20f45939f
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
2 files changed