Wrong file path for uhd_images_downloader.py

# Summary

`uhd_usrp_probe` tells me to download firmware files using
`/usr/lib/x86_64-linux-gnu/uhd/utils/uhd_images_downloader.py`
however, the file is actually located at:
```
$ apt-file search uhd_images_downloader.py
uhd-host: /usr/lib/uhd/utils/uhd_images_downloader.py
```

# System
```
$ lsb_release -rd
Description: KDE neon User Edition 5.16
Release: 18.04
```
(also observed on stock Ubuntu)

```
$ apt-cache policy uhd-host
uhd-host:
  Installed: 3.10.3.0-2
  Candidate: 3.10.3.0-2
  Version table:
 *** 3.10.3.0-2 500
        500 http://de.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        100 /var/lib/dpkg/status
```

# Detailed Description

After installation, firmware images must be downloaded from the Ettus servers. As far as I can see, there is no separate firmware package in the repositories.

Here, we run `uhd_usrp_probe` to discover the USRP devices:

```
$ uhd_usrp_probe
linux; GNU C++ version 7.3.0; Boost_106501; UHD_003.010.003.000-0-unknown

UHD Warning:
    EnvironmentError: IOError: Could not find path for image: usrp_b200_fw.hex
    Using images directory: <no images directory located>
    Set the environment variable 'UHD_IMAGES_DIR' appropriately or follow the below instructions to download the images package.
    Please run:
     "/usr/lib/x86_64-linux-gnu/uhd/utils/uhd_images_downloader.py"
Error: LookupError: KeyError: No devices found for ----->
Empty Device Address
```

Initializing the device fails because of the missing firmware files. The path for `uhd_images_downloader.py` is wrong and should point to `/usr/lib/uhd/utils/uhd_images_downloader.py`