Arduino Mega 1280 support

Fix ldflags

Update targets/arduino-mega1280.json

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

Update atmega1280.json

Update Makefile

targets/arduino-mega1280.json

@@ -0,0 +1,8 @@
+{
+    "inherits": ["atmega1280"],
+    "build-tags": ["arduino_mega1280"],
+    "ldflags": [
+        "-Wl,--defsym=_bootloader_size=4096"
+    ],
+    "flash-command":"avrdude -c arduino -b 57600 -p atmega1280 -P {port} -U flash:w:{hex}:i -v -D"
+}