buildman: Make -V (verbose_build) really be verbose

The help text for -V says we will pass V=1 but all it really did was not
pass in -s.  Change the logic to pass make V=1 with given to buildman -V or
-s to make otherwise.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Simon Glass <sjg@chromium.org>