AP_HAL_Linux: added SerialDevice interface

Adds the interface that will be used for encapsulating various mediums
that can be used in the Linux port.

libraries/AP_HAL_Linux/SerialDevice.h

@@ -0,0 +1,19 @@
+#ifndef __AP_HAL_LINUX_SERIALDEVICE_H__
+#define __AP_HAL_LINUX_SERIALDEVICE_H__
+
+#include <stdint.h>
+#include <stdlib.h>
+
+class SerialDevice {
+public: 
+    virtual ~SerialDevice() {}
+
+    virtual bool open() = 0;
+    virtual bool close() = 0;
+    virtual ssize_t write(const uint8_t *buf, uint16_t n) = 0;
+    virtual ssize_t read(uint8_t *buf, uint16_t n) = 0;
+    virtual void set_blocking(bool blocking) = 0;
+    virtual void set_speed(uint32_t speed) = 0;
+};
+
+#endif