LoongArch: Fix help text of CMDLINE_EXTEND in Kconfig

It is the built-in command line appended to the bootloader command line,
not the bootloader command line appended to the built-in command line.

Fixes: fa96b57c ("LoongArch: Add build infrastructure")
Signed-off-by: 谢致邦 (XIE Zhibang) <Yeking@Red54.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>