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_atmega.go

@@ -97,7 +97,7 @@ func (i2c *I2C) stop() {
 }
 
 // writeByte writes a single byte to the I2C bus.
-func (i2c *I2C) writeByte(data byte) {
+func (i2c *I2C) writeByte(data byte) error {
 	// Write data to register.
 	avr.TWDR.Set(data)
 
@@ -107,6 +107,7 @@ func (i2c *I2C) writeByte(data byte) {
 	// Wait till data is transmitted.
 	for !avr.TWCR.HasBits(avr.TWCR_TWINT) {
 	}
+	return nil
 }
 
 // readByte reads a single byte from the I2C bus.
@@ -190,7 +191,7 @@ func (uart *UART) handleInterrupt(intr interrupt.Interrupt) {
 }
 
 // WriteByte writes a byte of data to the UART.
-func (uart *UART) WriteByte(c byte) error {
+func (uart *UART) writeByte(c byte) error {
 	// Wait until UART buffer is not busy.
 	for !uart.statusRegA.HasBits(avr.UCSR0A_UDRE0) {
 	}
@@ -198,6 +199,8 @@ func (uart *UART) WriteByte(c byte) error {
 	return nil
 }
 
+func (uart *UART) flush() {}
+
 // SPIConfig is used to store config info for SPI.
 type SPIConfig struct {
 	Frequency uint32