targets: rename arduino target to arduino-uno

Rename the "arduino" target to "arduino-uno" to better reflect the
board it represents. The "arduino" name is kept as an alias that
inherits from "arduino-uno" for backward compatibility.

- Rename targets/arduino.json to targets/arduino-uno.json
- Create targets/arduino.json as alias inheriting from arduino-uno
- Rename board_arduino.go to board_arduino_uno.go
- Update build tag from "arduino" to "arduino_uno"
- Rename corresponding example files

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

src/machine/board_arduino_uno.go

@@ -0,0 +1,40 @@
+//go:build arduino_uno
+
+package machine
+
+// Digital pins, marked as plain numbers on the board.
+const (
+	D0  = PD0 // RX
+	D1  = PD1 // TX
+	D2  = PD2
+	D3  = PD3
+	D4  = PD4
+	D5  = PD5
+	D6  = PD6
+	D7  = PD7
+	D8  = PB0
+	D9  = PB1
+	D10 = PB2
+	D11 = PB3
+	D12 = PB4
+	D13 = PB5
+)
+
+// LED on the Arduino
+const LED Pin = D13
+
+// ADC on the Arduino
+const (
+	ADC0 Pin = PC0
+	ADC1 Pin = PC1
+	ADC2 Pin = PC2
+	ADC3 Pin = PC3
+	ADC4 Pin = PC4 // Used by TWI for SDA
+	ADC5 Pin = PC5 // Used by TWI for SCL
+)
+
+// UART pins
+const (
+	UART_TX_PIN Pin = PD1
+	UART_RX_PIN Pin = PD0
+)