Use debootstrap with --no-merged-usr by default

By default debootstrap creates systems with /usr being merged, leading
to issues like the xfsdump issue we saw in
grml#85, also dpkg -S ... and
further commands break with /usr being merged.

It's possible to generate custom basefiles with debootstrap, but by
default we should invoke debootstrap with the --no-merged-usr option,
otherwise we might have different ISO layouts, depending on whether we
use appropriate basefiles (as on our offical build host) vs. locally.

Closes: grml#86

grml-live

@@ -678,7 +678,7 @@ if [ -z "$FAI_DEBOOTSTRAP" ] ; then
 fi
 
 if [ -z "$FAI_DEBOOTSTRAP_OPTS" ] ; then
-  FAI_DEBOOTSTRAP_OPTS="--exclude=info,tasksel,tasksel-data --include=aptitude --arch $ARCH"
+  FAI_DEBOOTSTRAP_OPTS="--exclude=info,tasksel,tasksel-data --include=aptitude --arch $ARCH --no-merged-usr"
 fi
 
 # create backup of old (not yet automatically generated) config file