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>

include/bootdev.h

@@ -258,7 +258,7 @@ int bootdev_find_by_label(const char *label, struct udevice **devp,
  * @devp: returns the device found, on success
  * @method_flagsp: If non-NULL, returns any flags implied by the label
  * (enum bootflow_meth_flags_t), 0 if none. Unset if function fails
- * Return: 0 if OK, -EINVAL if the uclass is not supported by this board,
+ * Return: 0 if OK, -EPFNOSUPPORT if the uclass is not supported by this board,
  * -ENOENT if there is no device with that number
  */
 int bootdev_find_by_any(const char *name, struct udevice **devp,