cert_create: Introduce COT build option

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>

tools/cert_create/Makefile

@@ -10,6 +10,7 @@ V		?= 0
 DEBUG		:= 0
 BINARY		:= ${PROJECT}${BIN_EXT}
 OPENSSL_DIR	:= /usr
+COT		:= tbbr
 
 MAKE_HELPERS_DIRECTORY := ../../make_helpers/
 include ${MAKE_HELPERS_DIRECTORY}build_macros.mk
@@ -23,8 +24,12 @@ OBJECTS := src/cert.o \
            src/main.o \
            src/sha.o
 
-# TBBR chain of trust definitions.
-include src/tbbr/tbbr.mk
+# Chain of trust.
+ifeq (${COT},tbbr)
+  include src/tbbr/tbbr.mk
+else
+  $(error Unknown chain of trust ${COT})
+endif
 
 HOSTCCFLAGS := -Wall -std=c99