Implement HKDF.

See https://tools.ietf.org/html/rfc5869.

Change-Id: I6d012d8c38806df80aa44350d3a86eaff499ac05
Reviewed-on: https://boringssl-review.googlesource.com/2533
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/err.h b/include/openssl/err.h
index b9c48c3..ca1f6c4 100644
--- a/include/openssl/err.h
+++ b/include/openssl/err.h
@@ -403,6 +403,7 @@
   ERR_LIB_DIGEST,
   ERR_LIB_CIPHER,
   ERR_LIB_USER,
+  ERR_LIB_HKDF,
   ERR_NUM_LIBS
 };
 
@@ -442,6 +443,7 @@
 #define ERR_R_USER_LIB ERR_LIB_USER
 #define ERR_R_DIGEST_LIB ERR_LIB_DIGEST
 #define ERR_R_CIPHER_LIB ERR_LIB_CIPHER
+#define ERR_R_HKDF_LIB ERR_LIB_HKDF
 
 /* Global reasons. */
 #define ERR_R_FATAL 64