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.
diff --git a/include/mbedtls/cipher.h b/include/mbedtls/cipher.h
index 3ee2ab7..62b925e 100644
--- a/include/mbedtls/cipher.h
+++ b/include/mbedtls/cipher.h
@@ -145,6 +145,9 @@
     MBEDTLS_CIPHER_CAMELLIA_128_CCM,     /**< Camellia cipher with 128-bit CCM mode. */
     MBEDTLS_CIPHER_CAMELLIA_192_CCM,     /**< Camellia cipher with 192-bit CCM mode. */
     MBEDTLS_CIPHER_CAMELLIA_256_CCM,     /**< Camellia cipher with 256-bit CCM mode. */
+    MBEDTLS_CIPHER_AES_128_OFB,
+    MBEDTLS_CIPHER_AES_192_OFB,
+    MBEDTLS_CIPHER_AES_256_OFB
 } mbedtls_cipher_type_t;
 
 /** Supported cipher modes. */