Fixes:
http://autobuild.buildroot.net/results/cf7c4f360f5464c700788cc8299fd086544c80e8/build-end.log
Older GNU make versions don't like the explicit undefine. It isn't really
needed as ifdef handles undefined and defined-to-the-empty-string the same
way, so just drop the undefine logic.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit b8a1301e81)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>