Allow DFU_UTIL to be overridden from the environment.

By using ?= instead of = we can override for a couple of scenarios:

1 - Users that haven't setup their udev rules properly can add
```bash
export DFU_UTIL='sudo dfu-util'
```
2 - If you build dfu-util from the latest source, then you can use the -s :leave option:
```bash
export DFU_UTIL='dfu-util -s :leave'
```
and this will cause micropython to run automatically as soon its finished programming.