[crypto] Dual-license more selected DRBG files

Allow the use of the iPXE DRBG implementation in BSD-licensed
projects.

Requested-by: Sean Davis <dive@hq.endersgame.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/hmac.c

  * You can also choose to distribute this program under the terms of
  * the Unmodified Binary Distribution Licence (as given in the file
  * COPYING.UBDL), provided that you have satisfied its requirements.
+ *
+ * Alternatively, you may distribute this code in source or binary
+ * form, with or without modification, provided that the following
+ * conditions are met:
+ *
+ *  1. Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the above disclaimer.
+ *
+ *  2. Redistributions in binary form must reproduce the above
+ *     copyright notice, this list of conditions and the above
+ *     disclaimer in the documentation and/or other materials provided
+ *     with the distribution.
  */
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );