sandbox: Boot in U-Boot through the standard call

Most architectures use jump_to_image_no_args() to jump from SPL to U-Boot.
At present sandbox is special in that it jumps in its
spl_board_load_image() call. This is not strictly correct, and means that
sandbox misses out some parts of board_init_r(), just as calling
bloblist_finish(), for example.

Change spl_board_load_image() to just identify the filename to boot, and
implement jump_to_image_no_args() to actually jump to it.

Signed-off-by: Simon Glass <sjg@chromium.org>