experimentally add and build wireguard

I need it temporarily over the next few weeks. Let’s see how much maintenance
effort it is to keep it in.

cmd/rtr7-build-kernel/build.go

@@ -91,6 +91,10 @@ CONFIG_KEXEC_FILE=y
 
 # For apu2c4 watchdog
 CONFIG_SP5100_TCO=y
+
+# For WireGuard
+CONFIG_NET_UDP_TUNNEL=y
+CONFIG_WIREGUARD=y
 `
 
 func downloadKernel() error {

cmd/rtr7-rebuild-kernel/kernel.go

@@ -53,7 +53,9 @@ var dockerFileTmpl = template.Must(template.New("dockerfile").
 	}).
 	Parse(dockerFileContents))
 
-var patchFiles = []string{}
+var patchFiles = []string{
+	"wireguard.patch",
+}
 
 func copyFile(dest, src string) error {
 	out, err := os.Create(dest)