packagegroup-core-boot: revert to specifying sysvinit as default init manager
Don't follow DISTRO_FEATURES_INITMAN as that makes the packagegroups rebuild if
you switch init manager.
As in hybrid situations there's generally a clear primary and minimal init
manager choice, so change VIRTUAL-RUNTIME_init_manager to set the primary init
manager, and roll your own groups/images for the secondary.
Signed-off-by: Ross Burton <ross.burton@intel.com>
