Add it to the variable index
Misc improvements to the documentation for this variable
(From yocto-docs rev: fb460a7c7aba1ad5754806f9cfad9ef591420317)
Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>