os, runtime: enable os.Stdin for baremetal target

src/examples/echo2/echo2.go

@@ -0,0 +1,31 @@
+// This is a echo console running on the os.Stdin and os.Stdout.
+// Stdin and os.Stdout are connected to machine.Serial in the baremetal target.
+//
+// Serial can be switched with the -serial option as follows
+// 1. tinygo flash -target wioterminal -serial usb examples/echo2
+// 2. tinygo flash -target wioterminal -serial uart examples/echo2
+//
+// This example will also work with standard Go.
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+func main() {
+	fmt.Printf("Echo console enabled. Type something then press enter:\r\n")
+
+	scanner := bufio.NewScanner(os.Stdin)
+
+	for {
+		msg := ""
+		fmt.Scanf("%s\n", &msg)
+		fmt.Printf("You typed (scanf) : %s\r\n", msg)
+
+		if scanner.Scan() {
+			fmt.Printf("You typed (scanner) : %s\r\n", scanner.Text())
+		}
+	}
+}