Refactor and change CMAC interface

Change the CMAC interface to match the mbedtls_md_hmac_xxxx() interface. This
changes the overall design of the CMAC interface to make it more consistent with
the existing HMAC interface, and will allow incremental updates of input data
rather than requiring all data to be presented at once, which is what the
current interface requires.

include/mbedtls/cipher.h

@@ -176,6 +176,11 @@ enum {
  */
 typedef struct mbedtls_cipher_base_t mbedtls_cipher_base_t;
 
+/**
+ * CMAC context (opaque struct).
+ */
+typedef struct mbedtls_cmac_context_t mbedtls_cmac_context_t;
+
 /**
  * Cipher information. Allows cipher functions to be called in a generic way.
  */
@@ -241,6 +246,11 @@ typedef struct {
 
     /** Cipher-specific context */
     void *cipher_ctx;
+
+#if defined(MBEDTLS_CMAC_C)
+    /** CMAC Specific context */
+    mbedtls_cmac_context_t *cmac_ctx;
+#endif
 } mbedtls_cipher_context_t;
 
 /**