attiny: remove dummy UART

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

src/runtime/runtime_atmega.go

@@ -4,8 +4,17 @@ package runtime
 
 import (
 	"device/avr"
+	"machine"
 )
 
+func initUART() {
+	machine.UART0.Configure(machine.UARTConfig{})
+}
+
+func putchar(c byte) {
+	machine.UART0.WriteByte(c)
+}
+
 // Sleep for a given period. The period is defined by the WDT peripheral, and is
 // on most chips (at least) 3 bits wide, in powers of two from 16ms to 2s
 // (0=16ms, 1=32ms, 2=64ms...). Note that the WDT is not very accurate: it can