init-manager: define dev_manager for all init managers

init-manager: define dev_manager for all init managers

Currently VIRTUAL-RUNTIME_dev_manager is defined by some init managers
with "??=" and in packagegroup-core-boot with "?=".
This means that this variable is different in this package group and in
all other the recipes.
This was discovered when trying to use new feature INIT_MANAGER when
migrating to scarthgap and using systemd distro. After deleting all
VIRTUAL_RUNTIME providers defined in init-manager-systemd udev was
installed additionally via packagegroup-core-boot.

Having a distro settings overriden in single recipe is wrong and needs
to be corrected. Therefore let's define the setting in all init managers
and remove it from packagegroup-core-boot.

core-image-tiny-initramfs has a dilemma - use busybox-mdev even if
distro does not enable it in busybox recipe to keep the image tiny, or
rather not install it even if distro enables it in busybox.
This patch chooses the first option.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>