opkg: fix package state detection

Looking for installed packages needs to be more specific. It has to match the whole package name not only the beginning.

Example: installing the ```ip``` package.

In the previous version it would run more less following command (I'm skipping -q to show what I mean):

```
#opkg list-installed | grep ^ip
iperf - 2.0.5-1
iptables - 1.4.10-5
iptables-mod-conntrack-extra - 1.4.10-5
iptables-mod-filter - 1.4.10-5
iptables-mod-ipopt - 1.4.10-5
```
This means that it will find something although no ```ip``` is installed.