Add reboot=force

reboot=force doesn't wait for any other CPUs on reboot.  This is useful when
you really need a system to reboot on its own.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>