machine.UART refactor (#3832)

* add gosched calls to UART

* add UART.flush() stubs for all supported architectures

* add comment un uart.go on flush functionality

* uart.writeByte as base of UART usage

* fix NXP having duplicate WriteByte

* fix writeByte not returning error on some platforms

* add flush method for fe310 device

* check for error in WriteByte call to writeByte

src/machine/machine_esp32.go

@@ -314,7 +314,7 @@ func (uart *UART) Configure(config UARTConfig) {
 	uart.Bus.CLKDIV.Set(peripheralClock / config.BaudRate)
 }
 
-func (uart *UART) WriteByte(b byte) error {
+func (uart *UART) writeByte(b byte) error {
 	for (uart.Bus.STATUS.Get()>>16)&0xff >= 128 {
 		// Read UART_TXFIFO_CNT from the status register, which indicates how
 		// many bytes there are in the transmit buffer. Wait until there are
@@ -324,6 +324,8 @@ func (uart *UART) WriteByte(b byte) error {
 	return nil
 }
 
+func (uart *UART) flush() {}
+
 // Serial Peripheral Interface on the ESP32.
 type SPI struct {
 	Bus *esp.SPI_Type