Makefile: just one file per line to spot changes

This breaks the long file lines into one line per file item, so it's
easier to spot changes.  Also sorts the files alphabetically.
Sort debian install files.
Refactor FAI_ETC_DIR handling

Adds the new script library subroutines-tools for common functions
shared between various FIA tools.  This adds the get_cfdir function,
which processes the FAI_ETC_DIR env, when no config directory is set
and does some (already existing) sanity checks.
Refactor config file reading and default handling

Moves all default values for fai.conf and make-fai-nfsroot.conf
into a corresponding .defaults file.  Adds read_cfg to the
subroutines-tools, which reads config files from a directory and
includes the .defaults file.

The only exception is the FAI_ETC_DIR variable, which is handled
in the set_cfdir function.