fs/cpio: don't fail systems without /dev/null

We pass all our dracut configuration files via a config dir, but dracut
insists with having one config file. Because we do not want to have to
chose which file we pass (which would then have to be excluded from the
config dir), we just used an arbitrary empty file, and /dev/null seemed
to be a good candidate.

However, some build environments do not have a complete /dev, and may be
missing entries otherwise taken for granted, like /dev/null. This is
especially the case in constrained environments like containers.

Switch away from using /dev/null, and do create an actual empty file
that we can use as the dracut config file.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/3249828364



Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>