arm64: Kconfig: make CMDLINE_FORCE depend on CMDLINE

When building allmodconfig KCONFIG_ALLCONFIG=$(pwd)/arch/arm64/configs/defconfig
CONFIG_CMDLINE_FORCE gets enabled. Which forces the user to pass the
full cmdline to CONFIG_CMDLINE="...".

Rework so that CONFIG_CMDLINE_FORCE gets set only if CONFIG_CMDLINE is
set to something except an empty string.

Suggested-by: John Garry <john.garry@huawei.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>