cmd/elf.c: Add SPDX tag

Based on reading the text of the license comment this appears to be
the BSD-2-Clause license but with an imperfect word match as
BSD-2-Clause was not (as far as I recall) a common license choice at the
time the code was written.

Cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/cmd/elf.c b/cmd/elf.c
index ba06df0..036be5f 100644
--- a/cmd/elf.c
+++ b/cmd/elf.c
@@ -1,16 +1,7 @@
+// SPDX-License-Identifier: BSD-2-Clause
 /*
  * Copyright (c) 2001 William L. Pitts
  * All rights reserved.
- *
- * Redistribution and use in source and binary forms are freely
- * permitted provided that the above copyright notice and this
- * paragraph and the following disclaimer are duplicated in all
- * such forms.
- *
- * This software is provided "AS IS" and without any express or
- * implied warranties, including, without limitation, the implied
- * warranties of merchantability and fitness for a particular
- * purpose.
  */
 
 #include <common.h>