bootstd: Scan all bootdevs in a boot_targets entry (take 2)

When the boot_targets environment variable is used with the distro-boot
scripts, each device is included individually. For example, if there
are three mmc devices, then we will have something like:

   boot_targets="mmc0 mmc1 mmc2"

In contrast, standard boot supports specifying just the uclass, i.e.:

   boot_targets="mmc"

The intention is that this should scan all MMC devices, but in fact it
currently only scans the first.

Update the logic to handle this case, without required BOOTSTD_FULL to
be enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Date Huang <tjjh89017@hotmail.com>
Reported-by: Vincent Stehlé <vincent.stehle@arm.com>
Reported-by: Ivan Ivanov <ivan.ivanov@suse.com>
Tested-by: Ivan T.Ivanov <iivanov@suse.de>