ref-manual/variables.rst: expand IMAGE_OVERHEAD_FACTOR glossary entry

There's a (second) overhead factor applied in images generated with Wic,
and this is already documented in the .wks reference. However, the
IMAGE_OVERHEAD_FACTOR entry does not mention it, and by looking at the
partition sizes (e.g. with parted) one may find it confusing that they
don't match with the expected rootfs size (e.g. in a scenario where the
extra space is "0" and IMAGE_OVERHEAD_FACTOR="1.0").

This second overhead is already documented, though:
https://docs.yoctoproject.org/ref-manual/kickstart.html#command-part-or-partition

Mention the '--overhead-factor' option in the glossary entry and add a
reference to the wks documentation.

(From yocto-docs rev: b9040e20b015e9b02683ec3014e4ade5eb59d41a)

Signed-off-by: Joao Marcos Costa <joaomarcos.costa@bootlin.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index d559899c6e..18ead7d046 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -4166,6 +4166,12 @@ system and gives an overview of their function and contents.
       added to the image by using the :term:`IMAGE_ROOTFS_EXTRA_SPACE`
       variable.
 
+      When using Wic tool, beware that a second overhead factor is also applied.
+      This overhead value is defined by the ``--overhead-factor`` option, which
+      defaults to "1.3" when omitted. See the
+      :ref:`ref-manual/kickstart:command: part or partition` chapter in
+      :doc:`/ref-manual/kickstart` for details.
+
    :term:`IMAGE_PKGTYPE`
       Defines the package type (i.e. DEB, RPM or IPK) used by the
       OpenEmbedded build system. The variable is defined appropriately by