kbuild: Fix GNU make v3.80 compatibility

kbuild: Fix GNU make v3.80 compatibility

BugLink: http://bugs.launchpad.net/bugs/793702

commit 43f67c98161c65f1b2e3af3a9ce6741850072c06 upstream.

According to Documentation/Changes, the kernel should be buildable with
GNU make 3.80+.  Commit 88d7be031f9f975bb3f50a0b5ef3796a671e7edf (kbuild:
Use a single clean rule for kernel and external modules) introduced the
"$(or" construct, which requires make 3.81.  This causes "make clean" to
malfunction when it is used with external modules.

Replace "$(or" with an equivalent "$(if" expression, to restore backward
compatibility.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>