Add smoke tests for machine package

The machine package wasn't tested for every board. Therefore, add a new
serial-like test that also tries to import the machine package. This
should highlight potential issues in the future.

src/examples/machinetest/machinetest.go

@@ -0,0 +1,17 @@
+package main
+
+// This is the same as examples/serial, but it also imports the machine package.
+// It is used as a smoke test for the machine package (for boards that don't
+// have an on-board LED and therefore can't use examples/blinky1).
+
+import (
+	_ "machine" // smoke test for the machine package
+	"time"
+)
+
+func main() {
+	for {
+		println("hello world!")
+		time.Sleep(time.Second)
+	}
+}