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_mimxrt1062_uart.go

@@ -173,7 +173,7 @@ func (uart *UART) Sync() error {
 }
 
 // WriteByte writes a single byte of data to the UART interface.
-func (uart *UART) WriteByte(c byte) error {
+func (uart *UART) writeByte(c byte) error {
 	uart.startTransmitting()
 	for !uart.txBuffer.Put(c) {
 	}
@@ -181,6 +181,8 @@ func (uart *UART) WriteByte(c byte) error {
 	return nil
 }
 
+func (uart *UART) flush() {}
+
 // getBaudRateDivisor finds the greatest over-sampling factor (4..32) and
 // corresponding baud rate divisor (1..8191) that best partition a given baud
 // rate into equal intervals.