Introduce COT build option

Allows to select the chain of trust to use when the Trusted Boot feature
is enabled. This affects both the cert_create tool and the firmware
itself.

Right now, the only available CoT is TBBR.

Change-Id: I7ab54e66508a1416cb3fcd3dfb0f055696763b3d
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/Makefile b/Makefile
index bd52c0b..249a486 100644
--- a/Makefile
+++ b/Makefile
@@ -1009,7 +1009,7 @@
 
 .PHONY: ${CRTTOOL}
 ${CRTTOOL}:
-	${Q}${MAKE} PLAT=${PLAT} USE_TBBR_DEFS=${USE_TBBR_DEFS} --no-print-directory -C ${CRTTOOLPATH}
+	${Q}${MAKE} PLAT=${PLAT} USE_TBBR_DEFS=${USE_TBBR_DEFS} COT=${COT} --no-print-directory -C ${CRTTOOLPATH}
 	@${ECHO_BLANK_LINE}
 	@echo "Built $@ successfully"
 	@${ECHO_BLANK_LINE}