docs/system/riscv: sifive_u: Update S-mode U-Boot image build instructions

Currently, the documentation outlines the process for building the
S-mode U-Boot image using `make menuconfig` and manual actions within
the menuconfig UI. However, this approach is fragile due to Kconfig
options potentially changing across different releases. For example,
CONFIG_OF_PRIOR_STAGE has been replaced by CONFIG_BOARD since v2022.01
release, and CONFIG_TEXT_BASE has been moved to the 'General setup'
menu from the 'Boot options' menu in v2024.01 release.

This update aims to make the S-mode U-Boot image build instructions
future-proof. It leverages the 'config' script provided in the U-Boot
source tree to edit the .config file, followed by a `make olddefconfig`.

Validated with U-Boot v2024.01 release.

Signed-off-by: Bin Meng <bmeng@tinylab.org>

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240104071523.273702-1-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

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