ref-manual/variables.rst: document the INITRAMFS_MAXSIZE variable

This variable specifies the maximum allowed size
of the initramfs image in Kbytes.
Fixes [YOCTO #15797]

CC: Yoann Congal <yoann.congal@smile.fr>
CC: Randy MacLeod <randy.macleod@windriver.com>
CC: Antonin Godard <antonin.godard@bootlin.com>
(From yocto-docs rev: 123d1ec8e532751ac416c21e3eaa92fd904e2171)

Signed-off-by: Christos Gavros <gavrosc@yahoo.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 07bcdddaf71c76cdec186cf0ddd97a47eec54972)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/variables.rst

@@ -4576,6 +4576,20 @@ system and gives an overview of their function and contents.
       See the :term:`MACHINE` variable for additional
       information.
 
+   :term:`INITRAMFS_MAXSIZE`
+      Defines the maximum allowed size of the :term:`Initramfs` image in Kbytes.
+      The build will fail if the :term:`Initramfs` image size exceeds this value.
+
+      The :term:`Initramfs` image size undergoes several calculation steps before
+      being compared to :term:`INITRAMFS_MAXSIZE`.
+      These steps are the same as those used for :term:`IMAGE_ROOTFS_MAXSIZE`
+      and are described in detail in that entry.
+
+      Thus, :term:`INITRAMFS_MAXSIZE` is compared with the result of the calculations
+      and is independent of the final image type (e.g. compressed).
+      A default value for :term:`INITRAMFS_MAXSIZE` is set in
+      :oe_git:`meta/conf/bitbake.conf </openembedded-core/tree/meta/conf/bitbake.conf>`.
+
    :term:`INITRAMFS_MULTICONFIG`
       Defines the multiconfig to create a multiconfig dependency to be used by
       the :ref:`ref-classes-kernel` class.