Create /home/$USERNAME in RELEASE class with up2date files from /etc/skel

Until now we removed everything from /home/grml in the RELEASE
class. The grml-home init script from grml-etc then used to copy
files from /etc/skel to /home/grml during bootup. Instead lets do
this during ISO build time to speed up the boot process. To
ensure that no unwanted files are in /home/grml (actually:
/home/$USERNAME) we remove the directory, recreate it and then
copy all files from /etc/skel.

This fixes the problem that /home/grml is empty and has wrong
permissions if grml-home isn't executed (which is happening
with systemd nowadays where we don't execute grml-home during
bootup).

Closes grml/release-planning#7 @ GH