[Board] Adafruit Trinket (#333)

* Add support for Adafruit Trinket-M0 board

targets/atsamd21e18a.json

@@ -0,0 +1,15 @@
+{
+	"inherits": ["cortex-m"],
+	"llvm-target": "armv6m-none-eabi",
+	"build-tags": ["atsamd21e18", "atsamd21", "sam"],
+	"cflags": [
+		"--target=armv6m-none-eabi",
+		"-Qunused-arguments"
+	],
+	"ldflags": [
+		"-T", "targets/atsamd21.ld"
+	],
+	"extra-files": [
+		"src/device/sam/atsamd21e18a.s"
+	]
+}

targets/trinket-m0.json

@@ -0,0 +1,5 @@
+{
+    "inherits": ["atsamd21e18a"],
+    "build-tags": ["sam", "atsamd21e18a", "trinket_m0"],
+    "flash": "bossac -d -i -e -w -v -R --offset=0x2000 {hex}"
+}