x86, Kconfig: Add config switch for CPU0 hotplug

New config switch CONFIG_BOOTPARAM_HOTPLUG_CPU0 sets default state of whether
the CPU0 hotplug is on or off.

If the switch is off, CPU0 is not hotpluggable by default. But the CPU0 hotplug
feature can still be turned on by kernel parameter cpu0_hotplug at boot.

If the switch is on, CPU0 is always hotpluggable.

The default value of the switch is off.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1352835171-3958-3-git-send-email-fenghua.yu@intel.com


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>