attiny: remove dummy UART

I think it's better not to provide a UART0 global at all than one that
does nothing.

src/machine/machine_atmega.go

@@ -114,6 +114,12 @@ func (i2c I2C) readByte() byte {
 	return byte(avr.TWDR.Get())
 }
 
+// UART
+var (
+	// UART0 is the hardware serial port on the AVR.
+	UART0 = UART{Buffer: NewRingBuffer()}
+)
+
 // UART on the AVR.
 type UART struct {
 	Buffer *RingBuffer

src/machine/machine_attiny.go

@@ -2,23 +2,6 @@
 
 package machine
 
-// UART on the AVR is a dummy implementation. UART has not been implemented for ATtiny
-// devices.
-type UART struct {
-	Buffer *RingBuffer
-}
-
-// Configure is a dummy implementation. UART has not been implemented for ATtiny
-// devices.
-func (uart UART) Configure(config UARTConfig) {
-}
-
-// WriteByte is a dummy implementation. UART has not been implemented for ATtiny
-// devices.
-func (uart UART) WriteByte(c byte) error {
-	return nil
-}
-
 // Tx is a dummy implementation. I2C has not been implemented for ATtiny
 // devices.
 func (i2c I2C) Tx(addr uint16, w, r []byte) error {

src/machine/machine_avr.go

@@ -148,9 +148,3 @@ type I2C struct {
 
 // I2C0 is the only I2C interface on most AVRs.
 var I2C0 = I2C{}
-
-// UART
-var (
-	// UART0 is the hardware serial port on the AVR.
-	UART0 = UART{Buffer: NewRingBuffer()}
-)

src/machine/uart.go

@@ -1,4 +1,4 @@
-// +build avr esp nrf sam sifive stm32 k210 nxp
+// +build atmega esp nrf sam sifive stm32 k210 nxp
 
 package machine