Add define to allow multipart ccm to work

Add (internal only) define to config.h which allows the temporary
implementation of CCM to work, by removing the buffer zeroization on tag
fail when decrypting. This will obviously be removed when multipart CCM
is properaly implemented

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/library/ccm.c b/library/ccm.c
index 424ee77..d52e7b0 100644
--- a/library/ccm.c
+++ b/library/ccm.c
@@ -386,7 +386,9 @@
 
     if( diff != 0 )
     {
+#ifndef MBEDTLS_INTERNAL_CCM_NO_ZEROIZE_ON_TAG_FAIL
         mbedtls_platform_zeroize( output, length );
+#endif
         return( MBEDTLS_ERR_CCM_AUTH_FAILED );
     }