[cmdline] Fix "isset" command

Commit b5f5f73 ("[cmdline] Expand settings within each command-line
token individually") introduced a regression into the "isset" command:
it is now possible for command-line arguments to be empty strings, and
so "isset" cannot simply check for a non-empty argument list.

Restore previous behaviour by checking for the presence of any
non-empty arguments, rather than checking for a non-empty argument
list.

Reported-by: Nemtallah Daher <n.daher@csuohio.edu>
Tested-by: Nemtallah Daher <n.daher@csuohio.edu>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/exec.c

  */
 static int isset_exec ( int argc, char **argv ) {
 	struct isset_options opts;
+	int i;
 	int rc;
 
 	/* Parse options */
 	if ( ( rc = parse_options ( argc, argv, &isset_cmd, &opts ) ) != 0 )
 		return rc;
 
-	/* Return success iff any arguments exist */
-	return ( ( optind == argc ) ? -ENOENT : 0 );
+	/* Return success if any argument is non-empty */
+	for ( i = optind ; i < argc ; i++ ) {
+		if ( argv[i][0] != '\0' )
+			return 0;
+	}
+
+	return -ENOENT;
 }
 
 /** "isset" command */