chflags -R noschg /usr/obj/*
rm -rf /usr/obj/*
make buildworld
make buildkernel KERNCONF=YOUR_KERNEL_HERE

mount -u /
mount -a -t ufs
adjkerntz -i
swapon -a
make installkernel

mount -u /
mount -a -t ufs
adjkerntz -i
swapon -a
make installworld
mergemaster -v