spl: Kconfig: Move SPL_DISPLAY_PRINT to Kconfig

Move this option to Kconfig and tidy up existing uses. Also add a function
comment to the header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/include/spl.h b/include/spl.h
index 16f1ef4..f700955 100644
--- a/include/spl.h
+++ b/include/spl.h
@@ -122,6 +122,13 @@
  */
 int spl_start_uboot(void);
 
+/**
+ * spl_display_print() - Display a board-specific message in SPL
+ *
+ * If CONFIG_SPL_DISPLAY_PRINT is enabled, U-Boot will call this function
+ * immediately after displaying the SPL console banner ("U-Boot SPL ...").
+ * This function should be provided by the board.
+ */
 void spl_display_print(void);
 
 /* NAND SPL functions */