Makefile: add help target

Add help target to Makefile

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

Makefile

@@ -71,6 +71,18 @@ all: config.mk
 	for i in $(SUBDIRS); \
 	do echo; echo $$i; $(MAKE) $(MFLAGS) -C $$i; done
 
+help:
+	@echo "Make Targets:"
+	@echo " all                 - build binaries"
+	@echo " clean               - remove products of build"
+	@echo " distclean           - remove configuration and build"
+	@echo " install             - install binaries on local machine"
+	@echo " cscope              - build cscope database"
+	@echo " snapshot            - generate version number header"
+	@echo ""
+	@echo "Make Arguments:"
+	@echo " V=[0|1]             - set build verbosity level"
+
 config.mk:
 	sh configure $(KERNEL_INCLUDE)