Some Serial object fixes...

added availableForWrite - overwrite Print virtual - get it from ring buffer
fix peek and Read, that if 0 bytes read from ring buffer return -1

begin method appeared to have initial garbage in it.  so have it call ring_buf_reset on the rx ring buffer

cores/arduino/zephyrSerial.cpp

@@ -66,6 +66,10 @@ void arduino::ZephyrSerial::begin(unsigned long baud, uint16_t conf)
 
 	uart_configure(uart, &config);
 	uart_irq_callback_user_data_set(uart, arduino::ZephyrSerial::IrqDispatch, this);
+	k_sem_take(&rx.sem, K_FOREVER);
+	ring_buf_reset(&rx.ringbuf);
+	k_sem_give(&rx.sem);
+
 	uart_irq_rx_enable(uart);
 }
 
@@ -126,26 +130,37 @@ int arduino::ZephyrSerial::available()
 	return ret;
 }
 
+int arduino::ZephyrSerial::availableForWrite()
+{
+	int ret;
+
+	k_sem_take(&rx.sem, K_FOREVER);
+	ret = ring_buf_space_get(&rx.ringbuf);
+	k_sem_give(&rx.sem);
+
+	return ret;
+}
+
 int arduino::ZephyrSerial::peek()
 {
 	uint8_t data;
 
 	k_sem_take(&rx.sem, K_FOREVER);
-	ring_buf_peek(&rx.ringbuf, &data, 1);
+	uint32_t cb_ret = ring_buf_peek(&rx.ringbuf, &data, 1);
 	k_sem_give(&rx.sem);
 
-	return data;
+	return cb_ret? data : -1;
 }
 
 int arduino::ZephyrSerial::read()
 {
 	uint8_t data;
 
 	k_sem_take(&rx.sem, K_FOREVER);
-	ring_buf_get(&rx.ringbuf, &data, 1);
+	uint32_t cb_ret = ring_buf_get(&rx.ringbuf, &data, 1);
 	k_sem_give(&rx.sem);
 
-	return data;
+	return cb_ret? data : -1;
 }
 
 size_t arduino::ZephyrSerial::write(const uint8_t *buffer, size_t size)

cores/arduino/zephyrSerial.h

@@ -61,6 +61,7 @@ class ZephyrSerial : public HardwareSerial
 	size_t write(const uint8_t data) { return write(&data, 1); }
 	using Print::write; // pull in write(str) and write(buf, size) from Print
 	int available();
+  	int availableForWrite();
 	int peek();
 	int read();