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/make_helpers/build_macros.mk b/make_helpers/build_macros.mk
index cc51393..2cdbfa3 100644
--- a/make_helpers/build_macros.mk
+++ b/make_helpers/build_macros.mk
@@ -347,9 +347,9 @@
 $(BIN): $(ELF)
 	@echo "  BIN     $$@"
 	$$(Q)$$(OC) -O binary $$< $$@
-	@echo
+	@${ECHO_BLANK_LINE}
 	@echo "Built $$@ successfully"
-	@echo
+	@${ECHO_BLANK_LINE}
 
 .PHONY: bl$(1)
 bl$(1): $(BIN) $(DUMP)