bootstd: Rename bootdev_setup_sibling_blk()

This name is a little confusing since it suggests that it sets up the
sibling block device. In fact it sets up a bootdev for it. Rename the
function to make this clearer.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>