Seeed WioTerminal support (#1124)

* machine/wioterminal: add support for wioterminal board

targets/atsamd51p19a.json

@@ -0,0 +1,11 @@
+{
+	"inherits": ["cortex-m4"],
+	"build-tags": ["atsamd51p19", "atsamd51", "sam"],
+	"cflags": [
+		"-Qunused-arguments"
+	],
+	"linkerscript": "targets/atsamd51.ld",
+	"extra-files": [
+		"src/device/sam/atsamd51p19a.s"
+	]
+}

targets/wioterminal.json

@@ -0,0 +1,8 @@
+{
+    "inherits": ["atsamd51p19a"],
+    "build-tags": ["sam", "atsamd51p19a", "wioterminal"],
+    "flash-1200-bps-reset": "true",
+    "flash-method": "msd",
+    "msd-volume-name": "Arduino",
+    "msd-firmware-name": "firmware.uf2"
+}