Ubuntu – How to resolve SSL error during make … modules_install command

I'm new posting questions, but read a lot in the askubuntu forum. My knowledge about linux is not professional, but decent. I am an everyday user and run all my computers with xubuntu. Today I have a question/problem, I do not find help about in the forum or via the almighty google…

My problem:
I am setting up Xubuntu 16.04 on my new hardware. The driver for a power mgmt chip on the mainboard is not available in the source so I would have to add it myself. Name is ftsteutates. The source code and make file is provided by Fujitsu-Siemens. I trust the source and would like to avoid a political discussion about signing modules.

I unpack everything to a new folder and run without any errors:

make -C /lib/modules/$(uname -r)/build M=$(pwd) modules

Afterwards ftsteutates.ko is available in the folder and I run

make -C /lib/modules/$(uname -r)/build M=$(pwd) modules_install

and receive the error message:

root@HoSe:~/HomeServer/ftsteutates# make -C /lib/modules/$(uname -r)/build M=$(pwd) modules_install
make: Entering directory '/usr/src/linux-headers-4.4.0-36-generic'
  INSTALL /home/martin/HomeServer/ftsteutates/ftsteutates.ko
At main.c:222:
- SSL error:02001002:system library:fopen:No such file or directory: bss_file.c:175
- SSL error:2006D080:BIO routines:BIO_new_file:no such file: bss_file.c:178
sign-file: certs/signing_key.pem: No such file or directory
  DEPMOD  4.4.0-36-generic
make: Leaving directory '/usr/src/linux-headers-4.4.0-36-generic'

I get from the message, that the make command is trying to sign the module, but can't find the signing_key.pem.

I read a little about signing keys and found out that the flag

CONFIG_MODULE_SIG_ALL=yes 

in the config file of the kernel means that all modules will be signed automatically during the make … modules_install call. Obviously this is not working for my case and I do not get why.

Further the config file contains

CONFIG_SYSTEM_TRUSTED_KEYS='certs/signing_key.pem'

I checked for this file and it is not present anywhere in the system. As from the https://www.kernel.org/doc/Documentation/module-signing.txt I guess it should be there.

More over I do not know the background of the two SSL error messages or find anything about it via search.

At this point I am stuck. Can anyone give me a hint on what to read and how to proceed in resolving this?

Regards
Martin

EDIT:
I also tried it with an Xubuntu 15.10 installation on my laptop. Similar problem. Make … modules workes. On make … modules_install: No SSL errors and no complaining about the signing_key.pem, but the message is:

Can't read private key.