manuals: document SPLASH_IMAGES variable

(From yocto-docs rev: a30204ade74a93077017e3c5e1167102a0213983)

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

documentation/dev-manual/custom-distribution.rst

@@ -98,11 +98,12 @@ layer. The following steps provide some more detail:
 
    -  Add any image recipes that are specific to your distribution.
 
-   -  Add a ``psplash`` append file for a branded splash screen. For
-      information on append files, see the
-      ":ref:`dev-manual/layers:appending other layers metadata with your layer`"
-      section.
+   -  Add a ``psplash`` append file for a branded splash screen, using
+      the :term:`SPLASH_IMAGES` variable.
 
    -  Add any other append files to make custom changes that are
       specific to individual recipes.
 
+   For information on append files, see the
+   ":ref:`dev-manual/layers:appending other layers metadata with your layer`"
+   section.

documentation/ref-manual/variables.rst

@@ -7826,6 +7826,38 @@ system and gives an overview of their function and contents.
       for such applications to use. This variable is set by default to
       ``psplash``.
 
+   :term:`SPLASH_IMAGES`
+      This variable, used by the ``psplash`` recipe, allows to customize
+      the default splashscreen image.
+
+      Specified images in PNG format are converted to ``.h`` files by the recipe,
+      and are included in the ``psplash`` binary, so you won't find them in
+      the root filesystem.
+
+      To make such a change, it is recommended to customize the
+      ``psplash`` recipe in a custom layer. Here is an example structure for
+      an ``ACME`` board::
+
+          meta-acme/recipes-core/psplash
+          ├── files
+          │   └── logo-acme.png
+          └── psplash_%.bbappend
+
+      And here are the contents of the ``psplash_%.bbappend`` file in
+      this example::
+
+          SPLASH_IMAGES = "file://logo-acme.png;outsuffix=default"
+          FILESEXTRAPATHS:prepend := "${THISDIR}/files:"
+
+      You could even add specific configuration options for ``psplash``,
+      for example::
+
+          EXTRA_OECONF += "--disable-startup-msg --enable-img-fullscreen"
+
+      For information on append files, see the                                                                            
+      ":ref:`dev-manual/layers:appending other layers metadata with your layer`"
+      section.
+
    :term:`SRCREV_FORMAT`
       See :term:`bitbake:SRCREV_FORMAT` in the BitBake manual.