fai-debconf: use data from ${class}.debconf.

Source debconf data from ${class}.debconf files and use fai-debconf
in make-fai-nfsroot for the NFSROOT class.
mkramdisk: add verbose option (-v).

This suppresses all output messages and adds a verbose mode.
f-m-n: set explicit NFSROOT_ROOTDIR subdirectory.

Currently FAI implicitly expect the NFSROOT to be booted using the
live-initramfs tools and therefore expects the filesystem to be
rooted in the /live/filesystem.dir directory.

To support additional boot options (dracut, initramfs-tools netboot)
this adds the NFSROOT_ROOTDIR to make-fai-nfsroot.conf to specify the
filesystem root of the NFSROOT, defaulting to /live/filesystem.dir.

This also drops the special option '-l' for live initramfs.
f-m-n: optionally skip creation of base archive.

Adds '-N' option do skip the creation of base.gz / base.tar.xc
archives.
f-m-n: run gzip compression in the background.

As for xz, run the compression of the base.tgz archive as a
background job.
f-m-n: rename setup_dhcp to setup_tftp.
f-m-n: use umount_dirs in cleanup codepath.

Drops the special umount handling from cleanup codepath and simply
call umount_dirs. Additionally remove the broken NFSroot.
f-m-n: optionally symlink kernels into TFTP root.

Adds the option '-S' to symlink kernels into the TFTP root instead of
copying them. The symlinking also creates default symlinks to the
"newest" kernel.
f-m-n: allow specifying the debootstrap script.

Add a 3rd optional parameter to the FAI_DEBOOTSTRAP config, so
debootstrap can be called with a different script, if no script
exists for the suite.
f-m-n: verbosify command calls.

Use the verbose flag wherever possible.
f-m-n: consolidate mount and umount.

Introduces mount_dirs and makes umount_dirs reverse all changes done
by mount_dirs. This also moves the ramdisk setup and the Ubuntu
upstart workaround (divert) into these functions.
f-m-n: export additional variables.

Export some more variables, so these are available in the NFSroot
hooks.
f-m-n: reorganize functions.

Reorganize functions to seperate between initialization (debootstrap),
package update and configuration.
f-m-n: use run-parts to list hooks.
f-m-n: copy NFSroot_files tree into NFSroot.

Copy the file tree from /etc/fai/NFSroot_files into NFSroot.
f-m-n: rename NFSroot config directories.

Drop configureable NFSROOT_HOOK and use NFSROOT.files and
NFSROOT.hooks as fixed locations. This also moves the apt
configuration to NFSROOT.files/etc/apt.

As we copy all files from /etc/fai/NFSROOT.files, we just need the
special handling for APT keys and don't need the apt directory,
so rename directory path from apt/keys to apt-keys.