bitbake.conf: correct description of HOSTTOOLS_DIR

HOSTTOOLS_DIR contains symlinks to host tools, not copies

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit fb7692da)
Signed-off-by: Steve Sakoman <steve@sakoman.com>