x86/Kconfig: Make CMDLINE_OVERRIDE depend on non-empty CMDLINE

When trying to boot an allmodconfig kernel that is built with
KCONFIG_ALLCONFIG=$(pwd)/arch/x86/configs/x86_64_defconfig, it doesn't
boot since CONFIG_CMDLINE_OVERRIDE gets enabled and that requires the
user to pass the full cmdline to CONFIG_CMDLINE.

Change so that CONFIG_CMDLINE_OVERRIDE gets set only if CONFIG_CMDLINE
is set to something except an empty string.

 [ bp: touchup. ]

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200124114615.11577-1-anders.roxell@linaro.org