Fix "make modules" script bug under Mac OS X.

For Mac OS uses BSD sed, it doesn't support "\n". Then, "make modules"
command cannot work correctly under Mac OS. Using tr command to replace
sed command, it has the same behavior under both Mac OS and Linux.

Change-Id: I29e044cc66bf24f6459cbb33164fd9f55f772e3c

core/main.mk

@@ -737,7 +737,7 @@ clobber:
 modules:
 	@echo "Available sub-modules:"
 	@echo "$(call module-names-for-tag-list,$(ALL_MODULE_TAGS))" | \
-	      sed -e 's/  */\n/g' | sort -u | $(COLUMN)
+	      tr -s ' ' '\n' | sort -u | $(COLUMN)
 
 .PHONY: showcommands
 showcommands: