manuals: document VIRTUAL-RUNTIME variables

Document the convention to use variables prefixed by VIRTUAL_RUNTIME.
Add references to the new term where possible.

Another reason is that such variables are recommended
in a warning issued by meta/classes-global/insane.bbclass

(From yocto-docs rev: 11e1ba97edac979868f199e43c1004db6678044c)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Reported-by: Tim Orling <ticotimo@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/device-manager.rst

@@ -60,10 +60,10 @@ kernel.
 All devices created by ``devtmpfs`` will be owned by ``root`` and have
 permissions ``0600``.
 
-To have more control over the device nodes, you can use a device manager
-like ``udev`` or ``busybox-mdev``. You choose the device manager by
-defining the ``VIRTUAL-RUNTIME_dev_manager`` variable in your machine or
-distro configuration file. Alternatively, you can set this variable in
+To have more control over the device nodes, you can use a device manager like
+``udev`` or ``busybox-mdev``. You choose the device manager by defining the
+:term:`VIRTUAL-RUNTIME_dev_manager <VIRTUAL-RUNTIME>` variable in your machine
+or distro configuration file. Alternatively, you can set this variable in
 your ``local.conf`` configuration file::
 
    VIRTUAL-RUNTIME_dev_manager = "udev"

documentation/ref-manual/variables.rst

@@ -6750,11 +6750,11 @@ system and gives an overview of their function and contents.
 
       .. note::
 
-         A corresponding mechanism for virtual runtime dependencies
-         (packages) exists. However, the mechanism does not depend on any
-         special functionality beyond ordinary variable assignments. For
-         example, ``VIRTUAL-RUNTIME_dev_manager`` refers to the package of
-         the component that manages the ``/dev`` directory.
+         A corresponding mechanism for virtual runtime dependencies (packages)
+         exists. However, the mechanism does not depend on any special
+         functionality beyond ordinary variable assignments. For example,
+         :term:`VIRTUAL-RUNTIME_dev_manager <VIRTUAL-RUNTIME>` refers to the
+         package of the component that manages the ``/dev`` directory.
 
          Setting the "preferred provider" for runtime dependencies is as
          simple as using the following assignment in a configuration file::
@@ -9866,6 +9866,33 @@ system and gives an overview of their function and contents.
       Additionally, you should also set the
       :term:`USERADD_ERROR_DYNAMIC` variable.
 
+   :term:`VIRTUAL-RUNTIME`
+      :term:`VIRTUAL-RUNTIME` is a commonly used prefix for defining virtual
+      packages for runtime usage, typically for use in :term:`RDEPENDS`
+      or in image definitions.
+
+      An example is ``VIRTUAL-RUNTIME_base-utils`` that makes it possible
+      to either use BusyBox based utilities::
+
+         VIRTUAL-RUNTIME_base-utils = "busybox"
+
+      or their full featured implementations from GNU Coreutils
+      and other projects::
+
+         VIRTUAL-RUNTIME_base-utils = "packagegroup-core-base-utils"
+
+      Here are two examples using this virtual runtime package. The
+      first one is in :yocto_git:`initramfs-framework_1.0.bb
+      </poky/tree/meta/recipes-core/initrdscripts/initramfs-framework_1.0.bb?h=scarthgap>`::
+
+         RDEPENDS:${PN} += "${VIRTUAL-RUNTIME_base-utils}"
+
+      The second example is in the :yocto_git:`core-image-initramfs-boot
+      </poky/tree/meta/recipes-core/images/core-image-initramfs-boot.bb?h=scarthgap>`
+      image definition::
+
+         PACKAGE_INSTALL = "${INITRAMFS_SCRIPTS} ${VIRTUAL-RUNTIME_base-utils} base-passwd"
+
    :term:`VOLATILE_LOG_DIR`
       Specifies the persistence of the target's ``/var/log`` directory,
       which is used to house postinstall target log files.