spl: Jump to image at end of board_init_r

spl_board_prepare_for_boot() is not called before jumping/invoking atf,
optee, opensbi or linux images.

Jump to image at the end of board_init_r() to fix this.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>