Make hmac_ctx optional

Note from future self: actually md_init_ctx will be re-introduced with the
same signature later, and a new function with the additional argument will be
added.
diff --git a/ChangeLog b/ChangeLog
index 9f04b62..81abd5b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,7 @@
   * Support for DTLS 1.0 and 1.2 (RFC 6347).
 
 API Changes
+   * md_init_ctx() gained a new argument for optional hmac usage
    * Removed individual mdX_hmac and shaX_hmac functions (use generic
      md_hmac functions from md.h)
    * Change md_info_t into an opaque structure (use md_get_xxx() accessors).