arm: spl: Provide for a board-specific loader

Some boards have a special way of loading U-Boot that does not fit with
the existing SPL code. For example sunxi uses an 'FEL' mode where U-Boot
is loaded over USB. Add a CONFIG option and boot mode for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>