[FRAMEBUFFER] Clean screen at boot

[dreamos82]

The screenshot above shows Dreamos running on a real hardware, loaded through grub., As it can be seen, screen is not cleaned, so the grub background/text image is still visible.

Ideally before printing/drawing anything on the framebuffer the screen should be cleaned.
This should be done after the memory manager has been fully initialized, so we can make sure that the framebuffer is fully mapped in memory.