if the boot time takes too long (e.g. by initializing other devices and software stacks) it might be possible to see the content of the uninitialized framebuffer memory on the display.
In order to avoid that, you can either switch on the backlight after the system is initialized or you can clear the framebuffer memory. For that purpose you can add the following line at the beginning of EwInitDisplay (file ew_bsp_display.c):
/* Initialize given framebuffer */
memset((void*)FRAME_BUFFER_ADDR, 0, FRAME_BUFFER_WIDTH * FRAME_BUFFER_HEIGHT * FRAME_BUFFER_DEPTH );
Does this help?