It allows to select the desired chain of trust. Right now, only the TBBR
CoT is available.

At this stage, this build option only affects the tool itself. It is not
plugged into the rest of the build system yet. To use it:

 > make -C tools/cert_create COT=tbbr

Change-Id: I4484418f76d3c7b330d8653c978499a181534dcd
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>