Device-specific asserts

Similar to `device.log`, which prepends device debug tag to the log msg, it would be nice to have a set of `asserts` functions specific to a device.

This way we could simply do
```
device.assert_true(xxx, msg)
```

instead of having to do string formatting every time:
```
asserts.assert_true(xxx, '%s msg` % device.debug_tag)
```