Poll for BOOTSEL readiness instead of fixed sleep, load with -x

lib/picotool/picotool.go

@@ -3,10 +3,23 @@ package picotool
 import (
 	"fmt"
 	"os/exec"
+	"time"
 )
 
+func WaitForBootsel(serial string, timeout time.Duration) error {
+	deadline := time.Now().Add(timeout)
+	for time.Now().Before(deadline) {
+		cmd := exec.Command("picotool", "info", "--ser", serial)
+		if err := cmd.Run(); err == nil {
+			return nil
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+	return fmt.Errorf("device %s not found in BOOTSEL after %v", serial, timeout)
+}
+
 func Load(uf2Path string, serial string) error {
-	cmd := exec.Command("picotool", "load", uf2Path, "--ser", serial)
+	cmd := exec.Command("picotool", "load", uf2Path, "-x", "--ser", serial)
 	out, err := cmd.CombinedOutput()
 	if err != nil {
 		return fmt.Errorf("picotool load: %w\n%s", err, out)