ref-manual: uboot-sign: Support firmware property in FIT config

Add documentation for the UBOOT_FIT_CONF_FIRMWARE variable, which allows
users to specify an image to be assigned to the "firmware" property of
the FIT configuration node. This explicitly defines the primary image
to boot, instead of relying on the first entry in the "loadables" list.

(From yocto-docs rev: f72fc69e2d2a074d5eaf1cc36b30726de58e3ab5)

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/classes.rst

@@ -3426,6 +3426,8 @@ The variables used by this class are:
 -  :term:`UBOOT_FIT_USER_SETTINGS`: adds a user-specific snippet to the U-Boot
    Image Tree Source (ITS). Users can include their custom U-Boot Image Tree
    Source (ITS) snippet in this variable.
+-  :term:`UBOOT_FIT_CONF_FIRMWARE`: adds one image to the ``firmware`` property
+   of the configuration node.
 -  :term:`UBOOT_FIT_CONF_USER_LOADABLES`: adds one or more user-defined images
    to the ``loadables`` property of the configuration node.
 

documentation/ref-manual/variables.rst

@@ -10439,6 +10439,15 @@ system and gives an overview of their function and contents.
 
          do_compile[depends] += "trusted-firmware-a:do_deploy"
 
+   :term:`UBOOT_FIT_CONF_FIRMWARE`
+      Adds one image to the ``firmware`` property of the configuration node of
+      the U-Boot Image Tree Source (ITS). Sets the ``firmware`` property to
+      select the image to boot first::
+
+         UBOOT_FIT_CONF_FIRMWARE = "fwa"
+
+      If not set, the first entry in "loadables" is used to boot instead.
+
    :term:`UBOOT_FIT_CONF_USER_LOADABLES`
       Adds one or more user-defined images to the ``loadables`` property of the
       configuration node of the U-Boot Image Tree Source (ITS). This variable