docs: add CONVERSION_CMD definition

(From yocto-docs rev: 05d376ccc65510a7d636e2685fb92a48d4712b0b)

Signed-off-by: Aatir Manzur <aatrapps@gmail.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/migration-guides/migration-2.2.rst

@@ -442,7 +442,7 @@ The following miscellaneous changes have occurred:
 -  :ref:`ref-classes-image`: Renamed COMPRESS(ION) to CONVERSION. This change
    means that ``COMPRESSIONTYPES``, ``COMPRESS_DEPENDS`` and
    ``COMPRESS_CMD`` are deprecated in favor of ``CONVERSIONTYPES``,
-   ``CONVERSION_DEPENDS`` and ``CONVERSION_CMD``. The ``COMPRESS*``
+   ``CONVERSION_DEPENDS`` and :term:`CONVERSION_CMD`. The ``COMPRESS*``
    variable names will still work in the 2.2 release but metadata that
    does not need to be backwards-compatible should be changed to use the
    new names as the ``COMPRESS*`` ones will be removed in a future

documentation/migration-guides/migration-2.4.rst

@@ -301,7 +301,7 @@ The following are additional changes:
    likely be removed in the next Yocto Project release.
 
 -  The ``vmdk``, ``vdi``, and ``qcow2`` image file types are now used in
-   conjunction with the "wic" image type through ``CONVERSION_CMD``.
+   conjunction with the "wic" image type through :term:`CONVERSION_CMD`.
    Consequently, the equivalent image types are now ``wic.vmdk``,
    ``wic.vdi``, and ``wic.qcow2``, respectively.
 

documentation/migration-guides/migration-3.4.rst

@@ -252,7 +252,7 @@ Miscellaneous
 
 - The previously deprecated ``COMPRESS_CMD`` and
   ``CVE_CHECK_CVE_WHITELIST`` variables have been removed. Use
-  ``CONVERSION_CMD`` and ``CVE_CHECK_WHITELIST`` (replaced by
+  :term:`CONVERSION_CMD` and ``CVE_CHECK_WHITELIST`` (replaced by
   :term:`CVE_CHECK_IGNORE` in version 3.5) respectively
   instead.
 

documentation/ref-manual/variables.rst

@@ -1297,6 +1297,19 @@ system and gives an overview of their function and contents.
       the recipe will be skipped, and if the build system attempts to build
       the recipe then an error will be triggered.
 
+   :term:`CONVERSION_CMD`
+      This variable is used for storing image conversion commands.
+      Image conversion can convert an image into different objects like:
+
+      -   Compressed version of the image
+
+      -   Checksums for the image
+
+      An example of :term:`CONVERSION_CMD` from :ref:`image-types
+      <ref-classes-image_types>` class is::
+
+         CONVERSION_CMD:lzo = "lzop -9 ${IMAGE_NAME}${IMAGE_NAME_SUFFIX}.${type}"
+
    :term:`COPY_LIC_DIRS`
       If set to "1" along with the
       :term:`COPY_LIC_MANIFEST` variable, the