Revert "bootstd: Scan all bootdevs in a boot_targets entry"

This commit was intended to allow all bootdevs in each boot_targets
entry to be scanned. However it causes bad ordering with bootdevs, e.g.
scanning Ethernet bootdevs when it should be keeping to mmc.

Revert it so we can try another approach.

This reverts commit e824d0d0.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Ivan T.Ivanov <iivanov@suse.de>