Doc how kernel source tree is detected

And howto overwrite it on cmdlime by make:

 make kbuilddir=~/git/kernel/net-next/

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
time_bench: fix warning: multi-line comment

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>