spl: Convert spl_board_load_image() to use linker list

Add a linker list declaration for this method and remove the explicit
switch() code. Update existing users.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

include/spl.h

@@ -228,12 +228,4 @@ bool spl_was_boot_source(void);
  */
 int spl_dfu_cmd(int usbctrl, char *dfu_alt_info, char *interface, char *devstr);
 
-/**
- * Board-specific load method for boards that have a special way of loading
- * U-Boot, which does not fit with the existing SPL code.
- *
- * @return 0 on success, negative errno value on failure.
- */
-int spl_board_load_image(struct spl_boot_device *bootdev);
-
 #endif