os, runtime: enable os.Stdin for baremetal target

src/runtime/runtime_stm32f103.go

@@ -20,6 +20,18 @@ 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()
+}
+
 // initCLK sets clock to 72MHz using HSE 8MHz crystal w/ PLL X 9 (8MHz x 9 = 72MHz).
 func initCLK() {
 	stm32.FLASH.ACR.SetBits(stm32.FLASH_ACR_LATENCY_WS2)                          // Two wait states, per datasheet