ref-manual: expand documentation on image-buildinfo class

Expand the documentation on the image-buildinfo class, and add variable
glossary entries for IMAGE_BUILDINFO_FILE, IMAGE_BUILDINFO_VARS and
SDK_BUILDINFO_FILE (latter is new in 4.1).

(From yocto-docs rev: f69b2f614aaa56a19432538cb1adf33a441ea31d)

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 16 insertions, 2 deletions

diff --git a/documentation/ref-manual/classes.rst b/documentation/ref-manual/classes.rst
index 5e49613b2f..cc0d212130 100644
--- a/documentation/ref-manual/classes.rst
+++ b/documentation/ref-manual/classes.rst
@@ -881,8 +881,22 @@ Yocto Project Overview and Concepts Manual.
 ``image-buildinfo.bbclass``
 ===========================
 
-The ``image-buildinfo`` class writes information to the target
-filesystem on ``/etc/build``.
+The ``image-buildinfo`` class writes a plain text file containing
+build information to the target filesystem at ``${sysconfdir}/buildinfo``
+by default (as specified by :term:`IMAGE_BUILDINFO_FILE`.
+This can be useful for manually determining the origin of any given
+image. It writes out two sections:
+
+1. `Build Configuration`: a list of variables and their values (specified
+   by :term:`IMAGE_BUILDINFO_VARS`, which defaults to :term:`DISTRO` and
+   :term:`DISTRO_VERSION`)
+
+2. `Layer Revisions`: the revisions of all of the layers used in the
+   build.
+
+Additionally, when building an SDK it will write the same contents
+to ``/buildinfo`` by default (as specified by
+:term:`SDK_BUILDINFO_FILE`).
 
 .. _ref-classes-image_types: