bitbake.conf: add VOLATILE_LOG_DIR variable

bitbake.conf: add VOLATILE_LOG_DIR variable

The default value is "yes" which results in the /var/log being a link
pointing to /var/volatile/log which is on tmpfs.

Setting valid boolean false value ('no', 'n', 'false', 'f', '0') would make
/var/log to be a directory on persistent storage.

[YOCTO #6132]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>