generic-board: make show_board_info a weak function

Make show_board_info() a weak function which allows for custom board
specific implementations thereof.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Max Krummenacher <max.krummenacher@toradex.com>