ref-manual/variables.rst: document SPL_DTB_BINARY

This variable is part of uboot-sign but not documented.

(From yocto-docs rev: 999e81f17bf60e187e709368ede3965df19bee59)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 05eb461cb1da76ad9cbaf634da7f47447b3f6765)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/classes.rst

@@ -3316,6 +3316,9 @@ The variables used by this class are:
 -  :term:`SPL_SIGN_ENABLE`: enable signing the FIT image.
 -  :term:`SPL_SIGN_KEYDIR`: directory containing the signing keys.
 -  :term:`SPL_SIGN_KEYNAME`: base filename of the signing keys.
+-  :term:`SPL_DTB_BINARY`: Name of the SPL device tree binary. Can be set to an
+   empty string to indicate that no SPL should be created and added to the FIT
+   image.
 -  :term:`UBOOT_FIT_ADDRESS_CELLS`: ``#address-cells`` value for the FIT image.
 -  :term:`UBOOT_FIT_DESC`: description string encoded into the FIT image.
 -  :term:`UBOOT_FIT_GENERATE_KEYS`: generate the keys if they don't exist yet.

documentation/ref-manual/variables.rst

@@ -8449,6 +8449,11 @@ system and gives an overview of their function and contents.
       section in the Yocto Project Board Support Package Developer's Guide
       for additional information.
 
+   :term:`SPL_DTB_BINARY`
+      When inheriting the :ref:`ref-classes-uboot-sign` class, the
+      :term:`SPL_DTB_BINARY` variable contains the name of the SPL binary to be
+      compiled.
+
    :term:`SPL_MKIMAGE_DTCOPTS`
       Options for the device tree compiler passed to ``mkimage -D`` feature
       while creating a FIT image with the :ref:`ref-classes-uboot-sign`