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>

targets/arduino-uno.json

@@ -0,0 +1,11 @@
+{
+	"inherits": ["atmega328p"],
+	"build-tags": ["arduino_uno"],
+	"ldflags": [
+		"--defsym=_bootloader_size=512",
+		"--defsym=_stack_size=512"
+	],
+	"flash-command": "avrdude -c arduino -p atmega328p -P {port} -U flash:w:{hex}:i",
+	"serial-port": ["2341:0043", "2341:0001", "2a03:0043", "2341:0243"],
+	"emulator": "simavr -m atmega328p -f 16000000 {}"
+}

targets/arduino.json

@@ -1,11 +1,3 @@
 {
-	"inherits": ["atmega328p"],
-	"build-tags": ["arduino"],
-	"ldflags": [
-		"--defsym=_bootloader_size=512",
-		"--defsym=_stack_size=512"
-	],
-	"flash-command": "avrdude -c arduino -p atmega328p -P {port} -U flash:w:{hex}:i",
-	"serial-port": ["2341:0043", "2341:0001", "2a03:0043", "2341:0243"],
-	"emulator": "simavr -m atmega328p -f 16000000 {}"
+	"inherits": ["arduino-uno"]
 }