Arduino Mega 1280 support

Fix ldflags

Update targets/arduino-mega1280.json

Co-authored-by: Ayke <aykevanlaethem@gmail.com>

Update atmega1280.json

Update Makefile

targets/atmega1280.json

@@ -0,0 +1,17 @@
+{
+    "inherits": ["avr"],
+    "cpu": "atmega1280",
+    "build-tags": ["atmega1280", "atmega"],
+    "cflags": [
+        "-mmcu=atmega1280"
+    ],
+    "ldflags": [
+        "-mmcu=avr51",
+        "-Wl,--defsym=_stack_size=512"
+    ],
+    "linkerscript": "src/device/avr/atmega1280.ld",
+    "extra-files": [
+        "targets/avr.S",
+        "src/device/avr/atmega1280.s"
+    ]
+}