arm: mvebu: turris_mox: Use show_board_info()

We are printing board information in last_stage_init(), but U-Boot has
dedicated function, show_board_info(), for this.

Move code which prints board information (board version, serial number,
module topology, ...) to show_board_info().

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