manuals: document SPDX_PRETTY variable

Now that it has been backported to Kirkstone
https://git.yoctoproject.org/poky/commit/?h=kirkstone&id=99483cff5c358d847ceb077349321eb57d5143e4

(From yocto-docs rev: 5883e897c34f25401b358a597fb6e18d80f7f90b)

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

documentation/ref-manual/classes.rst

@@ -380,8 +380,8 @@ The toplevel :term:`SPDX` output file is generated in JSON format as a
 as well as in ``tmp/deploy/spdx``.
 
 The exact behaviour of this class, and the amount of output can be controlled
-by the :term:`SPDX_ARCHIVE_PACKAGED`, :term:`SPDX_ARCHIVE_SOURCES` and
-:term:`SPDX_INCLUDE_SOURCES` variables.
+by the :term:`SPDX_PRETTY`, :term:`SPDX_ARCHIVE_PACKAGED`,
+:term:`SPDX_ARCHIVE_SOURCES` and :term:`SPDX_INCLUDE_SOURCES` variables.
 
 See the description of these variables and the
 ":ref:`dev-manual/common-tasks:creating a software bill of materials`"

documentation/ref-manual/variables.rst

@@ -7165,6 +7165,17 @@ system and gives an overview of their function and contents.
       image), compared to just using the
       :ref:`create-spdx <ref-classes-create-spdx>` class with no option.
 
+   :term:`SPDX_PRETTY`
+      This option makes the SPDX output more human-readable, using
+      identation and newlines, instead of the default output in a
+      single line::
+
+         SPDX_PRETTY = "1"
+
+      The generated SPDX files are approximately 20% bigger, but
+      this option is recommended if you want to inspect the SPDX
+      output files with a text editor.
+
    :term:`SPDXLICENSEMAP`
       Maps commonly used license names to their SPDX counterparts found in
       ``meta/files/common-licenses/``. For the default :term:`SPDXLICENSEMAP`