added arduino-mkr1000 target

Author: fleshin

targets/arduino-mkr1000.json

@@ -0,0 +1,6 @@
+{
+    "inherits": ["atsamd21g18a"],
+    "build-tags": ["sam", "atsamd21g18a", "arduino_mkr1000"],
+    "flash-command": "bossac -i -e -w -v -R -U --port={port} --offset=0x2000 {bin}",
+    "flash-1200-bps-reset": "true"
+}