pentoo-installer: Change genkernel.conf when necessary

```
Problem:
The installer can encrypt the root / partition.
Running genkernel on such an installation currently creates an initramfs 
without luks/gpg support.

Solution:
Installer should modify /etc/genkernel.conf on target system.
```

Original issue reported on code.google.com by `Wuod...@gmail.com` on 4 Jan 2015 at 11:01
