defaultsetup.conf: enable select init manager

defaultsetup.conf: enable select init manager

Introduce a new variable INIT_MANAGER and create 4 init-manager-*.inc
files to configure init manager settings. Available values of
INIT_MANAGER are sysvinit, systemd, mdev-busybox and a default of none.
'none' provides backwards compatibility.

The settings of various VIRTUAL-RUNTIME variables are moved into these
files from the packagegroups.

[YOCTO #13031]

[Modifications by RP for backwards compatibility]

(From OE-Core rev: 8d0b4704a526a48cd5e67df61b613424bbbdccde)

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>