Created a new variable description in the glossary. This
variable is used in the layer.conf file and needed to be
documented.
(From yocto-docs rev: 5c514c0e90ca920bdf183b1f9449a06b906235ca)
Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>