boot: Allow FIT to fall back from best-match option

When the best-match feature fails to find something, use the provided
config name as a fallback. The allows SPL to select a suitable config
when best-match is enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/image.h

@@ -1411,7 +1411,9 @@ int fit_check_format(const void *fit, ulong size);
  * copied into the configuration node in the FIT image. This is required to
  * match configurations with compressed FDTs.
  *
- * Returns: offset to the configuration to use if one was found, -1 otherwise
+ * Returns: offset to the configuration to use if one was found, -EINVAL if
+ * there a /configurations or /images node is missing, -ENOENT if no match was
+ * found, -ENXIO if the FDT node has no compatible string
  */
 int fit_conf_find_compat(const void *fit, const void *fdt);