video: add an option to skip cfb console init

This patch add an option to skip cfb console init for boards
who want to show a logo, but not use the cfb console. This is
needed for the siemens boards, which have a bmp bootlogo, but
do not need the cfb console.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>
[agust: use '__weak int board_cfb_skip(void)']
Signed-off-by: Anatolij Gustschin <agust@denx.de>