spl: Convert spl_net_load_image() to use linker list

Add a linker list declaration for this method and remove the explicit
switch() code. We need two variants - one for BOOT_DEVICE_CPGMAC and one for
BOOT_DEVICE_USBETH.

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

include/spl.h

@@ -181,9 +181,6 @@ struct spl_image_loader {
 		.load_image = __method, \
 	}
 
-/* Ethernet SPL functions */
-int spl_net_load_image(struct spl_boot_device *bootdev);
-
 /* SPL FAT image functions */
 int spl_load_image_fat(struct blk_desc *block_dev, int partition,
 		       const char *filename);