kernel: fix build of kmod-udptunnel4 and kmod-udptunnel6

The kernel config option CONFIG_NET_UDP_TUNNEL is not visible and can
not directly be activated. When kmod-udptunnel4 or kmod-udptunnel6 are
build these packages could be empty when no other kernel module selects
CONFIG_NET_UDP_TUNNEL.

Reported-by: Baptiste Jonglez <baptiste@bitsofnetworks.org>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/generic/patches-3.18/263-udp_tunnel_kconfig_hack.patch

@@ -0,0 +1,11 @@
+--- a/net/ipv4/Kconfig
++++ b/net/ipv4/Kconfig
+@@ -308,7 +308,7 @@ config NET_IPVTI
+ 	  on top.
+ 
+ config NET_UDP_TUNNEL
+-	tristate
++	tristate "IP: UDP tunnel"
+ 	select NET_IP_TUNNEL
+ 	default n
+ 

target/linux/generic/patches-4.1/263-udp_tunnel_kconfig_hack.patch

@@ -0,0 +1,11 @@
+--- a/net/ipv4/Kconfig
++++ b/net/ipv4/Kconfig
+@@ -308,7 +308,7 @@ config NET_IPVTI
+ 	  on top.
+ 
+ config NET_UDP_TUNNEL
+-	tristate
++	tristate "IP: UDP tunnel"
+ 	select NET_IP_TUNNEL
+ 	default n
+ 

target/linux/generic/patches-4.4/263-udp_tunnel_kconfig_hack.patch

@@ -0,0 +1,11 @@
+--- a/net/ipv4/Kconfig
++++ b/net/ipv4/Kconfig
+@@ -308,7 +308,7 @@ config NET_IPVTI
+ 	  on top.
+ 
+ config NET_UDP_TUNNEL
+-	tristate
++	tristate "IP: UDP tunnel"
+ 	select NET_IP_TUNNEL
+ 	default n
+