Gh142 support debian

In case of Debian (Wheezy) this change will generate the following error message because the mirror debian-backports was not defined.

`
==> default: E: Unable to locate package linux-image-3.16.0-4-amd64
`

Even if the mirror is defined the kernel must be pinned or you use something like this

`$kernelpackage         = 'linux-image-3.16.0-4-amd64/wheezy-backports'`