[COMMANDS] Add blank, fillrect and a built-in logo framebuffer commands.

The following commands have been added for the framebuffer:
  - blank <fb_name> <value>
    . fb_name: The name of the framebuffer
    . value:   The integer value corresponding to the requested blanking state
               (FB_BLANK_POWERDOWN, FB_BLANK_VSYNC_SUSPEND,
                FB_BLANK_HSYNC_SUSPEND, FB_BLANK_NORMAL, FB_BLANK_UNBLANK).

  - fillrect <fb_name> <x> <y> <w> <h> <c> [<rop>]
    . fb_name: The name of the framebuffer
    . x:       The rectangle horizontal position.
    . y:       The rectangle vertical position.
    . w:       The rectangle width.
    . h:       The rectangle height.
    . rop:     The fill operation type (optional).

  - logo <fb_name> <x> <y> <w> <h>\n");
    . fb_name: The name of the framebuffer
    . x:       The logo horizontal position.
    . y:       The logo vertical position.
    . w:       The logo width.
    . h:       The logo height.

The added built-in logo is the one of the IRT SystemX.

Signed-off-by: Jimmy Durand Wesolowski <jimmy.durand-wesolowski@openwide.fr>