Generate make-fai-nfsroot.conf and nfsroot.conf on-the-fly

Generate make-fai-nfsroot.conf and nfsroot.conf on-the-fly

FAI 4 uses nfsroot.conf instead of make-fai-nfsroot.conf without
providing any backwards compatibility. :( So while at it rework
the way how make-fai-nfsroot.conf is handled:

* No longer ship /etc/grml/fai/make-fai-nfsroot.conf at all
* Generate nfsroot.conf based on default settings (can be overriden
  by $FAI_DEBOOTSTRAP + $FAI_DEBOOTSTRAP_OPTS) on the fly
* Provide symlink make-fai-nfsroot.conf pointing to nfsroot.conf
  to support old and new FAI versions at the same time
* After execution copy generated nfsroot.conf file to log directory
  and get rid of make-fai-nfsroot.conf and nfsroot.conf in
  $GRML_FAI_CONFIG (/etc/grml/fai/ by default) afterwards

Closes: https://github.com/grml/grml-live/issues/4