smp: limit the scope of some SMP-only functions

z_smp_init() is only available if CONFIG_SMP is defined,
smp_timer_init() also depends on two Kconfig parameters. Also make it
conditional in cavs_timer.c. Also clarify some SMP-related comments
there.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

kernel/include/kernel_internal.h

diff --git a/kernel/include/kernel_internal.h b/kernel/include/kernel_internal.h
index e55c73f..684d87d 100644
--- a/kernel/include/kernel_internal.h
+++ b/kernel/include/kernel_internal.h
@@ -136,9 +136,13 @@
 	thread->base.swap_data = data;
 }
 
+#ifdef CONFIG_SMP
 extern void z_smp_init(void);
 
+#if CONFIG_MP_NUM_CPUS > 1 && !defined(CONFIG_SMP_BOOT_DELAY)
 extern void smp_timer_init(void);
+#endif
+#endif
 
 extern void z_early_boot_rand_get(uint8_t *buf, size_t length);