Rename ssl_populate_transform() -> ssl_tls12_populate_transform()

In TLS 1.2 specific code, the internal helper functions
ssl_populate_transform() builds an SSL transform structure,
representing a specific record protection mechanism.

In preparation for a subsequent commit which will introduce
a similar helper function specific to TLS 1.3, this commmit
renames ssl_populate_transform() to ssl_tls12_populate_transform().

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index cc19f47..ca92d68 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -740,7 +740,8 @@
 
 #if defined(MBEDTLS_SSL_CONTEXT_SERIALIZATION)
     /* We need the Hello random bytes in order to re-derive keys from the
-     * Master Secret and other session info, see ssl_populate_transform() */
+     * Master Secret and other session info,
+     * see ssl_tls12_populate_transform() */
     unsigned char randbytes[64]; /*!< ServerHello.random+ClientHello.random */
 #endif /* MBEDTLS_SSL_CONTEXT_SERIALIZATION */
 };