spl: Convert boot_device into a struct

At present some spl_xxx_load_image() functions take a parameter and some
don't. Of those that do, most take an integer but one takes a string.

Convert this parameter into a struct so that we can pass all functions the
same thing. This will allow us to use a common function signature.

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

arch/sandbox/cpu/spl.c

@@ -38,7 +38,7 @@ void spl_board_announce_boot_device(void)
 	printf("%s\n", fname);
 }
 
-int spl_board_load_image(void)
+int spl_board_load_image(struct spl_boot_device *bootdev)
 {
 	char fname[256];
 	int ret;