UBUNTU: [Packaging] Fix config file assembly

Commit 'UBUNTU: [Packaging] dkms -- switch to a consistent build prefix
length and strip' introduced a helper tool fix-filenames which was added as
a prerequisite to the $(stampdir)/stamp-prepare-tree-% rule. This rule
generates the config file by concatenating all prereqs together
(cat $^ ...) including the fix-filenames binary blob which results in
warnings like:

scripts/kconfig/conf  --silentoldconfig Kconfig
.config:8536:warning: unexpected data: ^?ELF^B^A^A
.config:8073:warning: unexpected data:
.config:8537:warning: unexpected data:
.config:8538:warning: unexpected data:
.config:8539:warning: unexpected data:

This is certainly not what we want, fix it.

Ignore: yes
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>