Widgetsignatue

Process signature file in widget according to http://www.w3.org/TR/widgets-digsig/#signature-verification. Because there is no Tizen CA root in image, reference validation and signature validation on signature will be added when CA is ready.
Invert the logic to make it more readable:

```c++
if (ret != 0) {
    LOG(ERROR) << ...;
    return false;
}
return true;
```