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>
diff --git a/include/init.h b/include/init.h
index c781789..f2cd46d 100644
--- a/include/init.h
+++ b/include/init.h
@@ -332,6 +332,8 @@
 /* Show arch-specific information for the 'bd' command */
 void arch_print_bdinfo(void);
 
+int do_bdinfo(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[]);
+
 #endif	/* __ASSEMBLY__ */
 /* Put only stuff here that the assembler can digest */