sandbox: sdl: Add an option to double the screen size

sandbox: sdl: Add an option to double the screen size

On high-DPI displays U-Boot's LCD window can look very small. Add a
-K flag to expand it to make things easier to read, while still using
the existing resolution internally.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Anatolij Gustschin <agust@denx.de>