read-only filesystem when installing - aufs missing

[René Bleisch]

Hi,
I just ran into problems when I tried to install a xenial client with my 
new xenial-config.
The installation got stuck due to a read-only filesystem - similar to 
Erwanns (11.2.2016 "NFSROOT mounted on /") and Dirks case ( 27.5.2016 
"no writeable /tmp on PXE-Boot...").

As in Erwanns case, it seems to be due to the missing "aufs" option in 
the PXE-file.
But I wonder, why the aufs option is missing in my tftp-config - it is 
not added when I execute fai-chboot.
If I manually add the "aufs" option to the PXE-file, everything works as 
expected.

In the FAI-changelog, I found the following entry for fai 5.1:
     "fai-chboot: automatic test if aufs or overlayfs is needed"

I suspect, that in my case fai-chboot somehow decides that aufs is not 
required
=> how does "fai-chboot" make its decision on using aufs or overlayfs 
(based on which conditions)?
=> is there a command line option or fai.conf/nfsroot.conf option to 
enforce fai-chboot to use aufs? *

* I know, I could use a PXE-template file as I already do it in my 
jessie/wheezy FAI - but I rather would avoid this in this case

Regards
René

------------------
My fai-chboot command:
/usr/sbin/fai-chboot -C /etc/fai_xenial -IFv -k 
LOGREMOTEDIR=/var/log/fai_ubuntu -u nfs://phkup26/srv/fai/config_xenial 
kupmolucca

the resulting PXE-file:
---
default fai-generated

label fai-generated
kernel vmlinuz-3.16.0-4-amd64
append initrd=initrd.img-3.16.0-4-amd64 ip=dhcp 
root=nfs:phkup26:/srv/fai/nfsroot LOGREMOTEDIR=/var/log/fai_ubuntu 
FAI_FLAGS=verbose,sshd,createvt 
FAI_CONFIG_SRC=nfs://phkup26/srv/fai/config_xenial FAI_ACTION=install 
hostname=kupmolucca
---

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