all: add Arduino Nano support

src/machine/board_arduino_nano.go

@@ -0,0 +1,27 @@
+// +build arduino_nano
+
+package machine
+
+// Return the current CPU frequency in hertz.
+func CPUFrequency() uint32 {
+	return 16000000
+}
+
+// LED on the Arduino
+const LED Pin = 13
+
+// ADC on the Arduino
+const (
+	ADC0 Pin = 0
+	ADC1 Pin = 1
+	ADC2 Pin = 2
+	ADC3 Pin = 3
+	ADC4 Pin = 4 // Used by TWI for SDA
+	ADC5 Pin = 5 // Used by TWI for SCL
+)
+
+// UART pins
+const (
+	UART_TX_PIN Pin = 1
+	UART_RX_PIN Pin = 0
+)