polkit: enable and disable systemd/consolekit support explicitly

* people using sysvinit need to add something like
  PACKAGECONFIG = "${@base_contains('VIRTUAL-RUNTIME_init_manager', 'sysvinit', 'consolekit', 'systemd', d)}"
  or just
  PACKAGECONFIG = "consolekit"
  to their own .bbappend if they don't want to get systemd in their
  images.
* and for the rest it will enable systemd explititly

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>