x86: change boot_cpu_id to boot_cpu_physical_apicid

This is to match i386. The former name was cuter,
but the current is more meaningful and more general,
since cpu_id can be a logical id.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>