avr: unify GPIO pin/port code

All the AVRs that I've looked at had the same pin/port structure, with
the possible states being input/floating, input/pullup, low, and high
(with the same PORT/DDR registers). The main difference is the number of
available ports and pins. To reduce the amount of code and avoid
duplication (and thus errors) I decided to centralize this, following
the design used by the atmega2560 but while using a trick to save
tracking a few registers.

In the process, I noticed that the Pin.Get() function was incorrect on
the atmega2560 implementation. It is now fixed in the unified code.

src/machine/machine_avr.go

@@ -5,6 +5,7 @@ package machine
 import (
 	"device/avr"
 	"runtime/volatile"
+	"unsafe"
 )
 
 type PinMode uint8
@@ -14,6 +15,41 @@ const (
 	PinOutput
 )
 
+// In all the AVRs I've looked at, the PIN/DDR/PORT registers followed a regular
+// pattern: PINx, DDRx, PORTx in this order without registers in between.
+// Therefore, if you know any of them, you can calculate the other two.
+//
+// For now, I've chosen to let the PORTx register be the one that is returned
+// for each specific chip and to calculate the others from that one. Setting an
+// output port (done using PORTx) is likely the most common operation and the
+// one that is the most time critical. For others, the PINx and DDRx register
+// can trivially be calculated using a subtraction.
+
+// Configure sets the pin to input or output.
+func (p Pin) Configure(config PinConfig) {
+	port, mask := p.getPortMask()
+	// The DDRx register can be found by subtracting one from the PORTx
+	// register, as this appears to be the case for many (most? all?) AVR chips.
+	ddr := (*volatile.Register8)(unsafe.Pointer(uintptr(unsafe.Pointer(port)) - 1))
+	if config.Mode == PinOutput {
+		// set output bit
+		ddr.SetBits(mask)
+	} else {
+		// configure input: clear output bit
+		ddr.ClearBits(mask)
+	}
+}
+
+// Get returns the current value of a GPIO pin.
+func (p Pin) Get() bool {
+	port, mask := p.getPortMask()
+	// As noted above, the PINx register is always two registers below the PORTx
+	// register, so we can find it simply by subtracting two from the PORTx
+	// register address.
+	pin := (*volatile.Register8)(unsafe.Pointer(uintptr(unsafe.Pointer(port)) - 2)) // PINA, PINB, etc
+	return (pin.Get() & mask) > 0
+}
+
 // Set changes the value of the GPIO pin. The pin must be configured as output.
 func (p Pin) Set(value bool) {
 	if value { // set bits