esp8266: add support for this chip

Many thanks to cnlohr for the nosdk8266 project:
    https://github.com/cnlohr/nosdk8266

src/machine/board_nodemcu.go

@@ -0,0 +1,21 @@
+// +build nodemcu
+
+// Pinout for the NodeMCU dev kit.
+
+package machine
+
+// GPIO pins on the NodeMCU board.
+const (
+	D0 Pin = 16
+	D1 Pin = 5
+	D2 Pin = 4
+	D3 Pin = 0
+	D4 Pin = 2
+	D5 Pin = 14
+	D6 Pin = 12
+	D7 Pin = 13
+	D8 Pin = 15
+)
+
+// Onboard blue LED (on the AI-Thinker module).
+const LED = D4