Changed every memcpy to SCA equivalent mbedtls_platform_memcpy

This makes physical attacks more difficult.
diff --git a/library/platform_util.c b/library/platform_util.c
index db46fe9..6d3c9ef 100644
--- a/library/platform_util.c
+++ b/library/platform_util.c
@@ -211,7 +211,7 @@
 
     if( lt != NULL )
     {
-        memcpy( tm_buf, lt, sizeof( struct tm ) );
+        mbedtls_platform_memcpy( tm_buf, lt, sizeof( struct tm ) );
     }
 
 #if defined(MBEDTLS_THREADING_C)