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>

common/board_info.c

@@ -15,7 +15,7 @@ int __weak checkboard(void)
  * If the root node of the DTB has a "model" property, show it.
  * Then call checkboard().
  */
-int show_board_info(void)
+int __weak show_board_info(void)
 {
 #ifdef CONFIG_OF_CONTROL
 	DECLARE_GLOBAL_DATA_PTR;