cmd/tailscale/cli: add "configure jetkvm" subcommand

To write the init script.

And fix the JetKVM detection to work during early boot while the filesystem
and modules are still being loaded; it wasn't being detected on early boot
and then tailscaled was failing to start because it didn't know it was on JetKVM
and didn't modprobe tun.

Updates #16524

Change-Id: I0524ca3abd7ace68a69af96aab4175d32c07e116
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version/distro/distro.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"runtime"
 	"strconv"
+	"strings"
 
 	"tailscale.com/types/lazy"
 	"tailscale.com/util/lineiter"
@@ -103,12 +104,20 @@ func linuxDistro() Distro {
 		return Unraid
 	case have("/etc/alpine-release"):
 		return Alpine
-	case haveDir("/userdata/jetkvm") && haveDir("/sys/kernel/config/usb_gadget/jetkvm"):
+	case runtime.GOARCH == "arm" && isDeviceModel("JetKVM"):
 		return JetKVM
 	}
 	return ""
 }
 
+func isDeviceModel(want string) bool {
+	if runtime.GOOS != "linux" {
+		return false
+	}
+	v, _ := os.ReadFile("/sys/firmware/devicetree/base/model")
+	return want == strings.Trim(string(v), "\x00\r\n\t ")
+}
+
 func freebsdDistro() Distro {
 	switch {
 	case have("/etc/pfSense-rc"):