Externalized the raspbian apt repo into the sources.list file.
Improved apt configuration feedback during install.
Added the addition of the raspberrypi.org gpg key to the feedback.
Added iteration over *.list files to add them to /etc/apt/sources.list.d/.

When the keyword __RELEASE__ is present, replace it with the value of
$release.
Added a newline at end of file.
Added iteration over *.pref files to add them to /etc/apt/preferences.d/.

When the keyword __RELEASE__ is present, replace it with the value of $release.
Changed feedback to also include full path in list file iteration.
Added iteration over *.key files to add them to apt.
Improved feedback by checking whether things went right instead of assuming it.

Also added some comments to explain what is happening and some empty lines
for better readability.
Removed launching of a subshell in order to iterate over files.
Changed the check for a succesfull cp by checking it's result code.
Merge pull request #142 from diederikdehaas/externalize-apt-config-2

Externalize apt config 2