generic-board: allow showing custom board info

Allow showing custom board info from a checkboard() function being
implemented if CONFIG_CUSTOM_BOARDINFO is specified.  Previously the
device tree model was always displayed not taking any
CONFIG_CUSTOM_BOARDINFO into account.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Simon Glass <sjg@chromium.org>