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>