openvswitch: Re-add CONFIG_OPENVSWITCH_VXLAN

This readds the config option CONFIG_OPENVSWITCH_VXLAN to avoid a
hard dependency of OVS on VXLAN. It moves the VXLAN config compat
code to vport-vxlan.c and allows compliation as a module.

Fixes: 614732eaa12d ("openvswitch: Use regular VXLAN net_device device")
Fixes: 2661371ace96 ("openvswitch: fix compilation when vxlan is a module")
Cc: Pravin B Shelar <pshelar@nicira.com>
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/openvswitch/Makefile b/net/openvswitch/Makefile
index 38e0e14..6e1701d 100644
--- a/net/openvswitch/Makefile
+++ b/net/openvswitch/Makefile
@@ -15,5 +15,6 @@
 	vport-internal_dev.o \
 	vport-netdev.o
 
+obj-$(CONFIG_OPENVSWITCH_VXLAN)+= vport-vxlan.o
 obj-$(CONFIG_OPENVSWITCH_GENEVE)+= vport-geneve.o
 obj-$(CONFIG_OPENVSWITCH_GRE)	+= vport-gre.o