Redhat: 'clean build area' error message during dkms build/install

Redhat: 'clean build area' error message during dkms build/install

dkms invokes a make clean command before and after building the kernel
module. The make clean that is issued at the start of building results
in a nuisance error message because the Makefile doesn't yet exist

    Building module:
    cleaning build area...(bad exit status: 2)

In the dkms.conf file, built from within the openafs.spec, change the
command defined in the CLEAN statement to test for the existence of the
Makefile prior to running the actual make clean

Reviewed-on: https://gerrit.openafs.org/13460
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 10f2c469f45eece0e12573388ae66e392e2dff1c)

Change-Id: If3440b576ad62441bb6b970016fa9808b51abf70
Reviewed-on: https://gerrit.openafs.org/13479
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>