usb: add USB mass storage class support

src/examples/usb-storage/main.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"machine"
+	"machine/usb/msc"
+	"time"
+)
+
+func main() {
+	msc.Port(machine.Flash)
+
+	for {
+		time.Sleep(2 * time.Second)
+	}
+}