local.conf.sample.extended: prefer INIT_MANAGER

Since the INIT_MANAGER variable has been introduced, there is no need
to append the distro features and set the init manager manually.

Replace the busybox/mdev and systemd examples with the 4 values
currently supported for the INIT_MANAGER variable.

Signed-off-by: Vivien Didelot <vdidelot@pbsc.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>