spl: add support for alternative boot device

Introduce spl_boot_list array, which defines a list of boot devices
that SPL will try before hanging. By default this list will consist
of only spl_boot_device(), but board_boot_order() can be overridden
by board code to populate the array with custom values.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>