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>