The LOCKUP_DETECTOR configuration option split into the
SOFTLOCKUP_DETECTOR and HARDLOCKUP_DETECTOR configuration option some
time ago. The HARDLOCKUP_DETECTOR option is only working on some
architectures, but SOFTLOCKUP_DETECTOR should work everywhere. Replace
KERNEL_LOCKUP_DETECTOR with KERNEL_SOFTLOCKUP_DETECTOR.

LOCKUP_DETECTOR will be selected by SOFTLOCKUP_DETECTOR automatically.

Fixes: b951f53f ("build: Add additional kernel debug options")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>