board: developerbox: update old NOR flash layout build instruction

v2023.07 is the last version supporting old NOR flash layout
by default. The later versions of U-Boot, Developerbox is
configured to enable A/B update and new NOR Flash layout
by default.
This commit updates the documentation to pin the U-Boot
version for the old NOR flash layout. It is still useful
for the user wants to replace the factory default EDK II
firmware to U-Boot.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Acked-by: Jassi Brar <jaswinder.singh@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/doc/board/socionext/developerbox.rst b/doc/board/socionext/developerbox.rst
index aa7080e..596cf9f 100644
--- a/doc/board/socionext/developerbox.rst
+++ b/doc/board/socionext/developerbox.rst
@@ -42,9 +42,10 @@
 You can build U-Boot without any additinal source code.::
 
   cd u-boot
+  git checkout v2023.07
   export ARCH=arm64
   export CROSS_COMPILE=aarch64-linux-gnu-
-  make SynQuacer_defconfig
+  make synquacer_developerbox_defconfig
   make -j `noproc`
 
 Then, expand the binary to 1MB for preparing flash.::