Fixes [YOCTO #5861]
Added a note to the SDKMACHINE glossary variable stating that you
can't set the variable in the distribution configuration file and if
you do, it won't get recognized.
(From yocto-docs rev: 8cdc14e553586f6a754dfc3a91cc9741eb1784ee)
Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>