pyb: Add LED blink example.

pyb/example_blink.py

@@ -0,0 +1,13 @@
+import time
+from pyb import LED
+
+l = LED("left:amber")
+print(l.get())
+while 1:
+#    l.on()
+    l.toggle()
+    print(l.get())
+    time.sleep(1)
+#    l.off()
+#    print(l.get())
+#    time.sleep(1)