dev/ref-manual: Document INIT_MANAGER

The INIT_MANAGER variable was added in 3.0 but it seems we didn't get
around to documenting it yet. I have added a variable glossary entry and
made the basic adjustment of the "Using systemd Exclusively" section in
the dev manual, however I think the latter section still needs work.

(From yocto-docs rev: 602c0e8f770516256dab04edfd887377303c06bb)

Signed-off-by: Paul Eggleton <bluelightning@bluelightning.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

2 files changed, 21 insertions, 18 deletions

diff --git a/documentation/dev-manual/init-manager.rst b/documentation/dev-manual/init-manager.rst
index 0617fed516..10c4754e62 100644
--- a/documentation/dev-manual/init-manager.rst
+++ b/documentation/dev-manual/init-manager.rst
@@ -38,26 +38,14 @@ following sections.
 Using systemd Exclusively
 =========================
 
-Set these variables in your distribution configuration file as follows::
-
-   DISTRO_FEATURES:append = " systemd"
-   VIRTUAL-RUNTIME_init_manager = "systemd"
-
-You can also prevent the SysVinit distribution feature from
-being automatically enabled as follows::
-
-   DISTRO_FEATURES_BACKFILL_CONSIDERED = "sysvinit"
-
-Doing so removes any
-redundant SysVinit scripts.
-
-To remove initscripts from your image altogether, set this variable
-also::
+Set the :term:`INIT_MANAGER` variable in your distribution configuration
+file as follows::
 
-   VIRTUAL-RUNTIME_initscripts = ""
+   INIT_MANAGER = "systemd"
 
-For information on the backfill variable, see
-:term:`DISTRO_FEATURES_BACKFILL_CONSIDERED`.
+This will enable systemd and remove sysvinit components from the image.
+See ``meta/conf/distro/include/init-manager-systemd.inc`` for exact
+details on what this does.
 
 Using systemd for the Main Image and Using SysVinit for the Rescue Image
 ========================================================================
diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index bfdf1d1401..79d56dd704 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -3956,6 +3956,21 @@ system and gives an overview of their function and contents.
          even if the toolchain's binaries are strippable, there are other files
          needed for the build that are not strippable.
 
+   :term:`INIT_MANAGER`
+      Specifies the system init manager to use. Available options are:
+
+      -  ``sysvinit`` - System V init (default for poky)
+      -  ``systemd`` - systemd
+      -  ``mdev-busybox`` - mdev provided by busybox
+      -  ``none`` - no init manager
+
+      More concretely, this is used to include
+      ``conf/distro/include/init-manager-${INIT_MANAGER}.inc`` into the global
+      configuration. You can have a look at the ``conf/distro/include/init-manager-*.inc``
+      files for more information, and also the
+      ":ref:`dev-manual/init-manager:selecting an initialization manager`"
+      section in the Yocto Project Development Tasks Manual.
+
    :term:`INITRAMFS_DEPLOY_DIR_IMAGE`
       Indicates the deploy directory used by :ref:`ref-tasks-bundle_initramfs`
       where the :term:`INITRAMFS_IMAGE` will be fetched from. This variable is