spl: Correct priority selection for image loaders

At present the name of the image comes first in the linker-list symbol
used. This means that the name of the function sets the sort order, which
is not the intention.

Update it to put the boot-device type first, then the priority. This
produces the expected behaviour.

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