misc/collect.py: introduce variable {base} to fix download_url

misc/collect.py: introduce variable {base} to fix download_url

While scanning a directory for profile.json files of different releases,
the releases might be in different sub directories. But the collect.py
script only takes one download URL template for all overview.json files.

This commit introduces a variable {base} for the download URL parameter
that will be replaced by the sub directory that distinguish the location
of different release from each other.

Example path structure:
* ../tmp/releases/18.06.8/targets
* ../tmp/snapshot/targets

collect.py call:
```
./misc/collect.py scan "https://download.openwrt.org/{base}/{target}" ../tmp/ www/
```
In www/config.json:
```
versions: {'18.06.8': 'data/18.06.8/overview.json', 'SNAPSHOT': 'data/SNAPSHOT/overview.json'},
```

In www/data/18.06.8/overview.json:
```
"download_url": "https://download.openwrt.org/releases/18.06.8/targets/{target}",
```

In www/data/SNAPSHOT/overview.json:
```
"download_url": "https://download.openwrt.org/snapshots/targets/{target}"
```