all: refactor router code to use interface

So the code is more modular, easier to read/maintain.

internal/router/ntp/ntp.go

@@ -0,0 +1,26 @@
+package ntp
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"time"
+
+	"github.com/Control-D-Inc/ctrld/internal/router/nvram"
+	"tailscale.com/logtail/backoff"
+)
+
+func Wait() error {
+	// Wait until `ntp_ready=1` set.
+	b := backoff.NewBackoff("ntp.Wait", func(format string, args ...any) {}, 10*time.Second)
+	for {
+		out, err := nvram.Run("get", "ntp_ready")
+		if err != nil {
+			return fmt.Errorf("PreStart: nvram: %w", err)
+		}
+		if out == "1" {
+			return nil
+		}
+		b.BackOff(context.Background(), errors.New("ntp not ready"))
+	}
+}