ref-manual/variables.rst: document the FIT_CONF_PREFIX variable

Added by commit 7892ee3dc37d ("kernel-fitimage: allow overriding FIT
configuration prefix") in OE-Core, but never documented.

(From yocto-docs rev: 1b6a3610d5d24580de34b7821413b0f1ea52b508)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 860891492b96eb127af5e7bab6348fca12167c68)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/variables.rst

@@ -3339,6 +3339,10 @@ system and gives an overview of their function and contents.
 
       This variable is used in the :ref:`ref-classes-kernel-fitimage` class.
 
+   :term:`FIT_CONF_PREFIX`
+      When using the :ref:`ref-classes-kernel-fitimage`, this is the prefix
+      used for creating FIT configuration nodes. Its default value is "conf-".
+
    :term:`FIT_DESC`
       Specifies the description string encoded into a FIT image. The
       default value is set by the :ref:`ref-classes-kernel-fitimage` class as