arm: mvebu: turris_omnia: Use show_board_info()

We are printing board information in checkboard() function, which is
called from the default weak implementation of show_board_info().

Rename checkboard() to show_board_info(). This throws away the weak
implementation of show_board_info().

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>