[image] Fail "imgexec"/"boot" if the image to execute is ambiguous

If there is more than one loaded image, refuse to automatically select
the image to execute.  There are at least two possible cases, with
different "correct" answers:

1. User loads image A by mistake, then loads image B and types "boot".
   User wants to execute image B.

2. User loads image A, then loads image B (which patches image A), then
   types "boot".  User wants to execute image A.

If a user actually wants to load multiple images, they must explicitly
specify which image is to be executed.

src/hci/commands/image_cmd.c

@@ -407,7 +407,7 @@ static int imgexec_exec ( int argc, char **argv ) {
 	} else {
 		image = imgautoselect();
 		if ( ! image ) {
-			printf ( "No loaded images\n" );
+			printf ( "No (unique) loaded image\n" );
 			return 1;
 		}
 	}