x86/boot: Spell out "boot CPU" for BP

It's not obvious to everybody that BP stands for boot processor. At
least it was not for me. And BP is also a CPU register on x86, so it
is ambiguous. Spell out "boot CPU" everywhere instead.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Alok Kataria <akataria@vmware.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/smpboot.c

@@ -249,7 +249,7 @@ static void notrace start_secondary(void *unused)
 	/* otherwise gcc will move up smp_processor_id before the cpu_init */
 	barrier();
 	/*
-	 * Check TSC synchronization with the BP:
+	 * Check TSC synchronization with the boot CPU:
 	 */
 	check_tsc_sync_target();