ref-manual: add "KERNEL_IMAGE_STRIP_EXTRA_SECTIONS" to variables

Add this variable to the variables glossary, and add links to it and
back to the do_strip() task for completeness.

(From yocto-docs rev: cc4b7ffb3b2558ae796decbd216302e253addf02)

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit f43fc622d2fd6bc832a2993841b2020f86c6475c)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Paul Barker <paul@pbarker.dev>

documentation/ref-manual/tasks.rst

@@ -784,11 +784,19 @@ any that exceed that value, but fail only if none of them fit.
 ``do_strip``
 ------------
 
-If ``KERNEL_IMAGE_STRIP_EXTRA_SECTIONS`` is defined, this task strips
+If :term:`KERNEL_IMAGE_STRIP_EXTRA_SECTIONS` is defined, this task strips
 the sections named in that variable from ``vmlinux``. This stripping is
 typically used to remove nonessential sections such as ``.comment``
 sections from a size-sensitive configuration.
 
+Common sections to strip:
+
+-  ``.comment``: Holds compiler version strings and information
+
+-  ``.note.*``: Extra notes often left by the compiler
+
+-  ``.debug``: Debugging information
+
 .. _ref-tasks-validate_branches:
 
 ``do_validate_branches``

documentation/ref-manual/variables.rst

@@ -5131,6 +5131,13 @@ system and gives an overview of their function and contents.
 
       See :term:`KERNEL_ARTIFACT_NAME` for additional information.
 
+   :term:`KERNEL_IMAGE_STRIP_EXTRA_SECTIONS`
+      If this variable is set, it should contain the sections to be
+      stripped from the ``vmlinux`` image by the kernel-related
+      :ref:`ref-tasks-strip` task. As a simple example::
+
+         KERNEL_IMAGE_STRIP_EXTRA_SECTIONS = ".comment .note.* .debug"
+
    :term:`KERNEL_IMAGETYPE`
       The type of kernel to build for a device, usually set by the machine
       configuration files and defaults to "zImage". This variable is used