serial_irq: Add new generic/serial_irq.c code

Extract out common code from avr/serial.c, sam3x8e/serial.c, and
stm32f1/serial.c into a new generic/serial_irq.c file.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/generic/serial_irq.h

@@ -0,0 +1,11 @@
+#ifndef __GENERIC_SERIAL_IRQ_H
+#define __GENERIC_SERIAL_IRQ_H
+
+// callback provided by board specific code
+void serial_enable_tx_irq(void);
+
+// serial_irq.c
+void serial_rx_byte(uint_fast8_t data);
+int serial_get_tx_byte(uint8_t *pdata);
+
+#endif // serial_irq.h