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>
diff --git a/common/board_info.c b/common/board_info.c
index 6afe98e..bd5dcfa 100644
--- a/common/board_info.c
+++ b/common/board_info.c
@@ -17,7 +17,7 @@
  */
 int show_board_info(void)
 {
-#ifdef CONFIG_OF_CONTROL
+#if defined(CONFIG_OF_CONTROL) && !defined(CONFIG_CUSTOM_BOARDINFO)
 	DECLARE_GLOBAL_DATA_PTR;
 	const char *model;