common: spl: move image load to its own function

common: spl: move image load to its own function

Refactor spl image load code out of board_init_r and into its own
function. This is a preparation for supporting alternative boot
devices.

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