Complete implementation of printer serial transport

- Allow to configure the baudrate (it is set by the printer settings,
  but the transport didn't care)
- Implement reading from the serial port (some printers will need us to
  poll the status and the like, as there is usually no hardware flow
  control)

Change-Id: I70ba2566595d5dfa5eda3d518614db6514cb2398

headers/libs/print/libprint/GraphicsDriver.h

@@ -54,6 +54,11 @@ protected:
 			void	WriteSpoolChar(char c)
 						throw (TransportException);
 
+			void	ReadSpoolData(void* buffer, size_t size)
+						throw (TransportException);
+			int		ReadSpoolChar()
+						throw (TransportException);
+
 	static	void	ConvertToRGB24(const void* src, void* dst, int width,
 						color_space cs);
 	static	void	ConvertToGray(const void* src, void* dst, int width,

headers/libs/print/libprint/Transport.h

@@ -39,6 +39,8 @@ public:
 
 	void			Write(const void *buffer, size_t size)
 						throw (TransportException);
+	void			Read(void *buffer, size_t size)
+						throw (TransportException);
 	bool			CheckAbort() const;
 	bool			IsPrintToFileCanceled() const;
 	const string&	LastError() const;