[autoboot] Add an iPXE error URI to the "nothing to boot" message

Change the message "No filename or root path specified" to include an
iPXE error URI.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/usr/autoboot.c

 			rc = 0;
 		}
 	} else {
-		printf ( "No filename or root path specified\n" );
 		rc = -ENOENT;
+		printf ( "Nothing to boot: %s\n", strerror ( rc ) );
 	}
 
  err_san_describe: