avr: implement UART interface

Signed-off-by: Ron Evans <ron@hybridgroup.com>

src/examples/echo/echo.go

@@ -0,0 +1,37 @@
+// This is a echo console running on the device UART.
+// Connect using default baudrate for this hardware, 8-N-1 with your terminal program.
+package main
+
+import (
+	"machine"
+	"time"
+)
+
+func main() {
+	machine.UART0.Configure(machine.UARTConfig{})
+	machine.UART0.Write([]byte("Echo console enabled. Type something then press enter:\r\n"))
+
+	input := make([]byte, 64)
+	i := 0
+	for {
+		if machine.UART0.Buffered() > 0 {
+			data, _ := machine.UART0.ReadByte()
+
+			switch data {
+			case 13:
+				// return key
+				machine.UART0.Write([]byte("\r\n"))
+				machine.UART0.Write([]byte("You typed: "))
+				machine.UART0.Write(input[:i])
+				machine.UART0.Write([]byte("\r\n"))
+				i = 0
+			default:
+				// just echo the character
+				machine.UART0.WriteByte(data)
+				input[i] = data
+				i++
+			}
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+}