sandbox: make SDL window resizable

Without resizing the SDL window showed by

    ./u-boot -D -l

is not legible on a high resolution screen.

Allow resizing the window

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/sdl.c b/arch/sandbox/cpu/sdl.c
index 7dc3dab..d4dab36 100644
--- a/arch/sandbox/cpu/sdl.c
+++ b/arch/sandbox/cpu/sdl.c
@@ -127,7 +127,8 @@
 	sdl.pitch = sdl.width * sdl.depth / 8;
 	SDL_Window *screen = SDL_CreateWindow("U-Boot", SDL_WINDOWPOS_UNDEFINED,
 					      SDL_WINDOWPOS_UNDEFINED,
-					      sdl.vis_width, sdl.vis_height, 0);
+					      sdl.vis_width, sdl.vis_height,
+					      SDL_WINDOW_RESIZABLE);
 	if (!screen) {
 		printf("Unable to initialise SDL screen: %s\n",
 		       SDL_GetError());