avr: add Arduino Mega 2560

targets/arduino-mega2560.json

@@ -0,0 +1,8 @@
+{
+    "inherits": ["atmega2560"],
+    "build-tags": ["arduino_mega2560"],
+    "ldflags": [
+        "-Wl,--defsym=_bootloader_size=8192"
+    ],
+    "flash-command":"avrdude -c wiring -b 115200 -p atmega2560 -P {port} -U flash:w:{hex} -v -D"
+}

targets/atmega2560.json

@@ -0,0 +1,18 @@
+{
+    "inherits": ["avr"],
+    "llvm-target": "avr-atmel-none",
+    "cpu": "atmega2560",
+    "build-tags": ["atmega2560", "atmega"],
+    "cflags": [
+        "-mmcu=atmega2560"
+    ],
+    "ldflags": [
+        "-mmcu=avr6",
+        "-Wl,--defsym=_stack_size=512"
+    ],
+    "linkerscript": "src/device/avr/atmega2560.ld",
+    "extra-files": [
+        "targets/avr.S",
+        "src/device/avr/atmega2560.s"
+    ]
+}