kernel: Provide null device as logger target

Logger target is the screen by default. Normally this shouldn't
be a problem, because log messages should not occur after GUI
initialization, but in case some spurious message is generated
(e.g. from USB), the GUI screen would be destroyed. This can be
prevented by directing the logger to the new null device. To do
this one has to create a file cmdline.txt on the SD card with this
contents in the first line:

logdev=null

src/kernel.h

@@ -24,6 +24,7 @@
 #include <circle/actled.h>
 #include <circle/koptions.h>
 #include <circle/devicenameservice.h>
+#include <circle/nulldevice.h>
 #include <circle/screen.h>
 #include <circle/serial.h>
 #include <circle/exceptionhandler.h>
@@ -62,6 +63,7 @@ class CKernel
 	CActLED			m_ActLED;
 	CKernelOptions		m_Options;
 	CDeviceNameService	m_DeviceNameService;
+	CNullDevice		m_NullDevice;
 	CScreenDevice		m_Screen;
 	//CSerialDevice		m_Serial;
 	CExceptionHandler	m_ExceptionHandler;