flash: add -o flag support to save built binary (Fixes #4937) (#4942)

* Add -o flag support to flash command. Fixes #4937
* Remove empty outpath check in validateOutputFormat
---------
Co-authored-by: rdon <you@example.com>

main.go

@@ -340,8 +340,17 @@ func dirsToModuleRootAbs(maindir, modroot string) []string {
 	return dirs
 }
 
+// validateOutputFormat checks if the output file extension matches the expected format
+func validateOutputFormat(outpath, expectedExt string) error {
+	actualExt := filepath.Ext(outpath)
+	if actualExt != expectedExt {
+		return fmt.Errorf("output format %s does not match target format %s", actualExt, expectedExt)
+	}
+	return nil
+}
+
 // Flash builds and flashes the built binary to the given serial port.
-func Flash(pkgName, port string, options *compileopts.Options) error {
+func Flash(pkgName, port, outpath string, options *compileopts.Options) error {
 	config, err := builder.NewConfig(options)
 	if err != nil {
 		return err
@@ -390,13 +399,24 @@ func Flash(pkgName, port string, options *compileopts.Options) error {
 	if !options.Work {
 		defer os.RemoveAll(tmpdir)
 	}
-
+	// Validate output format before building
+	if outpath != "" {
+		if err := validateOutputFormat(outpath, fileExt); err != nil {
+			return err
+		}
+	}
 	// Build the binary.
 	result, err := builder.Build(pkgName, fileExt, tmpdir, config)
 	if err != nil {
 		return err
 	}
 
+	// Save output file if specified (after build, before flashing)
+	if outpath != "" {
+		if err := copyFile(result.Binary, outpath); err != nil {
+			return fmt.Errorf("failed to save output file: %v", err)
+		}
+	}
 	// do we need port reset to put MCU into bootloader mode?
 	if config.Target.PortReset == "true" && flashMethod != "openocd" {
 		port, err := getDefaultPort(port, config.Target.SerialPort)
@@ -1298,6 +1318,11 @@ extension at all.`
 			(https://tinygo.org/docs/reference/microcontrollers/).
 			Examples: "arduino-nano", "d1mini", "xiao".
 
+	-o={filename}:
+			Save the built binary to the specified output file. The file
+			format must match the target's expected format (e.g., .hex,
+			.uf2). Both flashing and saving will be performed.
+
 	-monitor: 
 			Start the serial monitor (see below) immediately after
 			flashing. However, some microcontrollers need a split second
@@ -1628,7 +1653,7 @@ func main() {
 		flag.BoolVar(&flagTest, "test", false, "supply -test flag to go list")
 	}
 	var outpath string
-	if command == "help" || command == "build" || command == "test" {
+	if command == "help" || command == "build" || command == "test" || command == "flash" {
 		flag.StringVar(&outpath, "o", "", "output filename")
 	}
 
@@ -1779,7 +1804,7 @@ func main() {
 	case "flash", "gdb", "lldb":
 		pkgName := filepath.ToSlash(flag.Arg(0))
 		if command == "flash" {
-			err := Flash(pkgName, *port, options)
+			err := Flash(pkgName, *port, outpath, options)
 			printBuildOutput(err, *flagJSON)
 		} else {
 			if !options.Debug {