bitbake.conf: store cooker logs in subdirectory of LOG_DIR

bitbake.conf: store cooker logs in subdirectory of LOG_DIR

* we should probably wait until
  http://git.openembedded.org/bitbake/commit/?id=1e06d8012868ba5a31503dc99cbf18570be629d9
  is in next required bitbake version, but error message if the directory doesn't exist is quite clear
* people with new enough bitbake could just change this in their local.conf

(From OE-Core rev: a1e4d32ede287f73c6acc796b43fe5b90c072fab)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>