image: Explicitly declare do_bdinfo()

Compiler is not happy:

common/image-board.c: In function ‘boot_get_kbd’:
common/image-board.c:902:17: warning: implicit declaration of function ‘do_bdinfo’ [-Wimplicit-function-declaration]
  902 |                 do_bdinfo(NULL, 0, 0, NULL);
      |                 ^~~~~~~~~

Move the forward declaration to a header.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

boot/image-board.c

@@ -14,6 +14,7 @@
 #include <env.h>
 #include <fpga.h>
 #include <image.h>
+#include <init.h>
 #include <mapmem.h>
 #include <rtc.h>
 #include <watchdog.h>