Implement egldisplay_t

Renamed KONFIG_linux into Linux.
Now you can use #include STR(C-kern/api/platform/KONFIG_OS/sysgraphicegl.h)
to include any platform specific header files.

Introduced C-kern/api/platform/Linux/graphic/sysegl.h
which contains platform specific include files for EGL support.

Extended test_errortimer_t with function IFERROR_testerrortimer.

Corrected all errors reported by
> C-kern/test/run_statictest.sh -v
Implement eglwindow_t

An eglwindow_t wraps a native window (X11 window)
into an EGL window surface.
This surface can be used as target for drawing
commands issued by OpenGL or OpenGL ES.

glxwindow_t is removed.
Instead EGL is the preferred way to initialize OpenGL.
If necessary GLX is reintroduced later but with
a new interface which integrates with surface_config_t
the same way EGL does.