The INIT_MANAGER variable was added in 3.0 but it seems we didn't get
around to documenting it yet. I have added a variable glossary entry and
made the basic adjustment of the "Using systemd Exclusively" section in
the dev manual, however I think the latter section still needs work.
(From yocto-docs rev: 602c0e8f770516256dab04edfd887377303c06bb)
Signed-off-by: Paul Eggleton <bluelightning@bluelightning.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>