Changed every memcpy to SCA equivalent mbedtls_platform_memcpy

This makes physical attacks more difficult.
diff --git a/library/entropy.c b/library/entropy.c
index 281ed23..75421cf 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -424,7 +424,7 @@
     for( i = 0; i < ctx->source_count; i++ )
         ctx->source[i].size = 0;
 
-    memcpy( output, buf, len );
+    mbedtls_platform_memcpy( output, buf, len );
 
     ret = 0;