lcd: add option for board specific splash screen preparation

Currently there is no logical place to put the code that prepares the
splash image data. The splash image data should be ready in memory
before bmp_display() is called, and after the environment is ready
(since lcd.c looks for the splash image in an address specified by
the environment variable "splashimage").

Our window of opportunity in board_init_r() is therefore: between
env_relocate() and bmp_display(), and from the available options
only the lcd related functions in drv_lcd_init() seem appropriate
for such lcd oriented code.

Add the option to prepare the splash image data in lcd_logo() right
before it is sent to be displayed.

Cc: Anatolij Gustschin <agust@denx.de>
Cc: Jeroen Hofstee <jeroen@myspectrum.nl>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>