examples/gnrc_border_router: enable ethernet uplink

Boards with an ethernet interface already have this as their `netdev_default`
so we don't have to enable anything here.

examples/gnrc_border_router/Makefile

@@ -11,8 +11,8 @@ RIOTBASE ?= $(CURDIR)/../..
 UPLINK ?= ethos
 
 # Check if the selected Uplink is valid
-ifeq (,$(filter ethos slip cdc-ecm wifi,$(UPLINK)))
-  $(error Supported uplinks are `ethos`, `slip`, `cdc-ecm` and `wifi`)
+ifeq (,$(filter ethos slip cdc-ecm wifi ethernet,$(UPLINK)))
+  $(error Supported uplinks are `ethos`, `slip`, `cdc-ecm`, `ethernet` and `wifi`)
 endif
 
 # Set the SSID and password of your WiFi network here
@@ -43,7 +43,7 @@ USEMODULE += ps
 #USEMODULE += gnrc_ipv6_nib_dns     # include RDNSS option handling
 
 # When using a WiFi uplink we should use DHCPv6
-ifeq (wifi,$(UPLINK))
+ifneq (,$(filter wifi ethernet,$(UPLINK)))
   USE_DHCPV6 ?= 1
 else
   USE_DHCPV6 ?= 0