Merge branch 'hkdf-support'

diff --cc ChangeLog
index 24bd92a9b91b783a5effdfe03b68327ef7ecdbc7,145376c051508bc96d8037a1200b5076c86879f1..10167a7117d5ee70963d5a96fba0067d5aa84ada
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--- 2/ChangeLog
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+ 2017-09-06  Niels Möller  <nisse@lysator.liu.se>
+ 
+       * hkdf.c (hkdf_expand): Eliminate a (signed) ssize_t variable, use
+       break rather than return at loop termination.
+ 
+ 2017-09-06  Niels Möller  <nisse@lysator.liu.se>
+ 
+       HKDF implementation, contributed by Nikos Mavrogiannopoulos.
+       * hkdf.c (hkdf_extract, hkdf_expand): New file, new functions.
+       * hkdf.h: New file.
+       * Makefile.in (nettle_SOURCES): Add hkdf.c.
+       (HEADERS): Add hkdf.h.
+       * testsuite/hkdf-test.c: Tests for hkdf-sha256 and hkdf-sha1.
+       * testsuite/Makefile.in (TS_NETTLE_SOURCES): Added hkdf-test.c.
+       * nettle.texinfo (Key derivation functions): Document HKDF.
+ 
 +2017-09-04  Andreas Schneider  <asn@samba.org>
 +
 +      * fat-arm.c: Add missing define for _GNU_SOURCE.
 +
  2017-08-27  Niels Möller  <nisse@lysator.liu.se>
  
        * configure.ac (GMP_NUMB_BITS): Set to dummy value "n/a" in