RISC-V: Kconfig: Fix help text of CMDLINE_EXTEND

RISC-V: Kconfig: Fix help text of CMDLINE_EXTEND

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: 3aed8c43267e ("RISC-V: Update Kconfig to better handle CMDLINE")
Signed-off-by: 谢致邦 (XIE Zhibang) <Yeking@Red54.com>
Link: https://lore.kernel.org/r/tencent_A93C7FB46BFD20054AD2FEF4645913FF550A@qq.com
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>