documentation.conf: sync with the Yocto Project reference manual
Make sure variables listed in the Glossary section of the reference
manual are listed here. There are still some listed here that aren't in
the manual; this should be fixed in the manual.
Signed-off-by: Belen Barros Pena <belen.barros.pena@intel.com> Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
