os, runtime: enable os.Stdin for baremetal target

src/runtime/runtime_rp2040.go

@@ -58,6 +58,18 @@ func putchar(c byte) {
 	machine.Serial.WriteByte(c)
 }
 
+func getchar() byte {
+	for machine.Serial.Buffered() == 0 {
+		Gosched()
+	}
+	v, _ := machine.Serial.ReadByte()
+	return v
+}
+
+func buffered() int {
+	return machine.Serial.Buffered()
+}
+
 // machineInit is provided by package machine.
 func machineInit()