Added a new variable description for SDK_INCLUDE_TOOLCHAIN.
Along with the new glossary entry, I updated a couple places
where it would be relevant to cross-reference the new variable.
(From yocto-docs rev: df99b097595e7488a93354082b8d9f33acd9c229)
Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>