DF-4327 harness build on guest (sf_callout_harness.ko)
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$ cd /root/kmod/sfcal && make SYSDIR=/usr/src/sys
sfcal=0  (cc ... -Werror ... -c sf_callout_harness.c; link ok)
-rw-r--r-- 1 root wheel 8096 Sep 19 12:31 /root/kmod/sfcal/sf_callout_harness.ko
$ kldload /root/kmod/sfcal/sf_callout_harness.ko
LOADED=0
sfcal: loaded (kern.sfcal.arm / kern.sfcal.stop)