bootstd: Report missing labels only when asked

bootstd: Report missing labels only when asked

Use the -l flag to indicate whether to report missing uclasses.

Also try to be more helpful when no devices are found. For example, when
we see something 'scsi0' requested and nothing was found, this indicates
that there are no SCSI devices, so show a suitable message.

Move messages out of the low-level functions so that silent operation
is possible.

This means they are never reported unless BOOTSTD_FULL is enabled, since
the -l flag cannot otherwise be set.

Suggested-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Simon Glass <sjg@chromium.org>