doc: correct references to overlay-fdt-boot.txt

doc/uImage.FIT/overlay-fdt-boot.txt does not exist anymore.
Reference the correct section of doc/usage/fit/overlay-fdt-boot.rst.

Fixes: 6f6e8bb695 ("doc: Bring in the FIT overlay information")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/README.pxe

@@ -160,7 +160,7 @@ kernel <path>	    - if this label is chosen, use tftp to retrieve the kernel
 		      It will passed to bootm with that address.
 		      (see: doc/uImage.FIT/command_syntax_extensions.txt)
 		      It useful for overlay selection in pxe file
-		      (see: doc/uImage.FIT/overlay-fdt-boot.txt)
+		      (see: doc/usage/fit/overlay-fdt-boot.rst).
 
 fdtoverlays <path> [...] - if this label is chosen, use tftp to retrieve the DT
                       overlay(s) at <path>. it will be temporarily stored at the

doc/usage/fdt_overlays.rst

@@ -76,8 +76,8 @@ There are two ways to apply overlays in U-Boot.
 * Manually load and apply overlays
 
 The remainder of this document will discuss using overlays via the manual
-approach. For information on using overlays as part of a FIT image please see:
-doc/uImage.FIT/overlay-fdt-boot.txt
+approach. For information on using overlays as part of a FIT image please see
+:ref:`fit_configuration_using_overlays`.
 
 Manually Loading and Applying Overlays
 --------------------------------------

doc/usage/fit/overlay-fdt-boot.rst

@@ -111,6 +111,8 @@ Where config is one of::
 
 This selects the DTB to use when booting.
 
+.. _fit_configuration_using_overlays:
+
 Configuration using overlays
 ----------------------------