examples/gnrc_networking: optional RPL and router

Introduce a Makefile variable for enabling RPL and/or routing
functionality.

examples/gnrc_networking/Makefile

@@ -7,14 +7,22 @@ BOARD ?= native
 # This has to be the absolute path to the RIOT base directory:
 RIOTBASE ?= $(CURDIR)/../..
 
+# By default this application will run as a host, i.e., not forward any
+# packets. Setting this variable to 1 will change this behavior and let the
+# node act as a router.
+ROUTER ?= 0
 # Include packages that pull up and auto-init the link layer.
 # NOTE: 6LoWPAN will be included if IEEE802.15.4 devices are present
 USEMODULE += netdev_default
 USEMODULE += auto_init_gnrc_netif
 # Activate ICMPv6 error messages
 USEMODULE += gnrc_icmpv6_error
-# Specify the mandatory networking module for a IPv6 host node
+# Specify the mandatory networking module for a IPv6 in router or host node
+ifeq (1,$(ROUTER))
+USEMODULE += gnrc_ipv6_router_default
+else
 USEMODULE += gnrc_ipv6_default
+endif
 # Additional networking modules that can be dropped if not needed
 USEMODULE += gnrc_icmpv6_echo
 USEMODULE += shell_cmd_gnrc_udp
@@ -24,7 +32,16 @@ USEMODULE += shell_cmds_default
 USEMODULE += ps
 USEMODULE += netstats_l2
 USEMODULE += netstats_ipv6
+
+# Optionally include the RPL routing protocol (particular useful if not all
+# nodes are within radio transmission range to the border router)
+USE_RPL ?= 0
+
+ifeq (1,$(USE_RPL))
+USEMODULE += gnrc_rpl
+USEMODULE += auto_init_gnrc_rpl
 USEMODULE += netstats_rpl
+endif
 
 # Optionally include DNS support. This includes resolution of names at an
 # upstream DNS server and the handling of RDNSS options in Router Advertisements