esp32: add support for basic GPIO

GPIO is much more advanced on the ESP32, but this is a starting point.
It gets examples/blinky1 to work.

src/machine/board_esp32-wroom-32.go

@@ -0,0 +1,6 @@
+// +build esp32_wroom_32
+
+package machine
+
+// Blue LED on the ESP32-WROOM-32 module.
+const LED = Pin(2)

src/machine/machine_esp32.go

@@ -13,7 +13,52 @@ const (
 	PinInput
 )
 
-func (p Pin) Set(value bool)
+// Configure this pin with the given configuration.
+func (p Pin) Configure(config PinConfig) {
+	if config.Mode == PinOutput {
+		// Set the 'output enable' bit.
+		if p < 32 {
+			esp.GPIO.ENABLE_W1TS.Set(1 << p)
+		} else {
+			esp.GPIO.ENABLE1_W1TS.Set(1 << (p - 32))
+		}
+	} else {
+		// Clear the 'output enable' bit.
+		if p < 32 {
+			esp.GPIO.ENABLE_W1TC.Set(1 << p)
+		} else {
+			esp.GPIO.ENABLE1_W1TC.Set(1 << (p - 32))
+		}
+	}
+}
+
+// Set the pin to high or low.
+// Warning: only use this on an output pin!
+func (p Pin) Set(value bool) {
+	if value {
+		if p < 32 {
+			esp.GPIO.OUT_W1TS.Set(1 << p)
+		} else {
+			esp.GPIO.OUT1_W1TS.Set(1 << (p - 32))
+		}
+	} else {
+		if p < 32 {
+			esp.GPIO.OUT_W1TC.Set(1 << p)
+		} else {
+			esp.GPIO.OUT1_W1TC.Set(1 << (p - 32))
+		}
+	}
+}
+
+// Get returns the current value of a GPIO pin when the pin is configured as an
+// input.
+func (p Pin) Get() bool {
+	if p < 32 {
+		return esp.GPIO.IN.Get()&(1<<p) != 0
+	} else {
+		return esp.GPIO.IN1.Get()&(1<<(p-32)) != 0
+	}
+}
 
 var (
 	UART0 = UART{Bus: esp.UART0, Buffer: NewRingBuffer()}