install_mirror_package function for Ubuntu/Debian: fetch package from a different release.

We are fetching deb packages from newer releases in 2 targets (`xephyr` with `xserver-xephyr` and `touch` with `touchegg`). The `audio` target will also require 2 of such packages (namely `libsbc1` and `libsbc-dev`), so I guess its time to put the code in a function.