kernel: make lwtunnel support optional

Not everyone will want to bloat their kernel by 24 kiB for such a niche
feature.

Fixes: a1a7f3274e0ed27511d45f62ee20281d8d57c7af "kernel: enable SRv6 support by
enabling lwtunnel"

Signed-off-by: Rui Salvaterra <[email protected]>

config/Config-kernel.in

@@ -975,9 +975,10 @@ if KERNEL_IPV6
 		def_bool n
 
 	config KERNEL_IPV6_SEG6_LWTUNNEL
-		def_bool y if !SMALL_FLASH
+		bool "Enable support for lightweight tunnels"
+		default y if !SMALL_FLASH
 		help
-		  Using lwtunnel requires full-ip package.
+		  Using lwtunnel (needed for IPv6 segment routing) requires ip-full package.
 
 	config KERNEL_LWTUNNEL_BPF
 		def_bool n