Update path to packages repo

Update the correct path to packages uri and rename it from snapshots to packages.

This also fix the following annoying error:
```
Updated list of available packages in /var/opkg-lists/base.
Downloading http://downloads.openwrt.org/snapshots/trunk/x86_64/packages/Packages.gz.
wget: server returned error: HTTP/1.1 404 Not Found
```