Add cipher abstraction and test cases for OFB block mode

Adds OFB as additional block mode in the cipher abstraction, and additional
test cases for that block mode.

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 7c23ffc..aff915e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -18,6 +18,8 @@
      ECDH primitive functions (mbedtls_ecdh_gen_public(),
      mbedtls_ecdh_compute_shared()) are supported for now. Contributed by
      Nicholas Wilson (#348).
+   * Add additional block mode, OFB (Output Feedback), to the AES module and
+     cipher abstraction module.
 
 API Changes
    * Add function mbedtls_net_poll to public API allowing to wait for a