conf: Switch to systemd by default and simplify init manager selection

conf: Switch to systemd by default and simplify init manager selection

This change effectively switches things to use INIT_MANAGER to select the init
system and drops the old compatibility 'none' method. The init manager selection
is now complex enough that requiring users to select it makes sense.

The new default is systemd, which reflects popular opinion. This is known to have
issues in some of our configurations such as musl but is also frequently asked for.

Anyone replacing defaultsetup.conf in their own setup will need to provide
equivalent functionality but that is execpted for any of the settings in there.

This change drops sysvinit from the default distro features backfill, meaning
we no longer need to remove it in systemd setups and places the init managers
on a more equal and equivalent standing.

This is a behaviour change for anyone using nodistro and anyone not already
setting INIT_MANAGER explictly. This does not change the default for distros
which select an init manager already (e.g. poky).

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>