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.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 2a49642..4546d59 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1674,7 +1674,8 @@
 /**
  * \def MBEDTLS_CMAC_C
  *
- * Enable the CMAC mode for block ciphers.
+ * Enable the CMAC (Cipher-based Message Authentication Code) mode for block
+ * ciphers.
  *
  * Module:  library/cmac.c
  *