Build: Correct Unix specific echo commands

Some recent changes have added direct use of the echo command without
parameters.  This fails on a Windows shell, because echo without
parameters reports the mode ("ECHO is on").
This is corrected using the ECHO_BLANK_LINE macro already provided
for that purpose.

Change-Id: I5fd7192861b4496f6f46b4f096e80a752cd135d6
Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>
diff --git a/Makefile b/Makefile
index d0568d9..984fbda 100644
--- a/Makefile
+++ b/Makefile
@@ -653,18 +653,18 @@
 ifneq (${GENERATE_COT},0)
 fwu_certificates: ${FWU_CRT_DEPS} ${CRTTOOL}
 	${Q}${CRTTOOL} ${FWU_CRT_ARGS}
-	@echo
+	@${ECHO_BLANK_LINE}
 	@echo "Built $@ successfully"
 	@echo "FWU certificates can be found in ${BUILD_PLAT}"
-	@echo
+	@${ECHO_BLANK_LINE}
 endif
 
 ${BUILD_PLAT}/${FWU_FIP_NAME}: ${FWU_FIP_DEPS} ${FIPTOOL}
 	${Q}${FIPTOOL} create ${FWU_FIP_ARGS} $@
 	${Q}${FIPTOOL} info $@
-	@echo
+	@${ECHO_BLANK_LINE}
 	@echo "Built $@ successfully"
-	@echo
+	@${ECHO_BLANK_LINE}
 
 fiptool: ${FIPTOOL}
 fip: ${BUILD_PLAT}/${FIP_NAME}