kernel: SMP timer integration

In SMP, the system timer is used for timeslicing on auxiliary CPUs,
but the base system timekeeping via _nano_sys_clock_tick_announce() is
still done on CPU0 only (because the framework isn't prepared for
asynchronous notification yet).  Skip processing on CPU1+.

Also, due to a hardware interaction* that is difficult to work around,
timer initialization on the auxiliary CPUs is done at the very end of
the CPU bringup, just before the swap into the scheduler.  A
smp_timer_init() API has been added for this purpose.

* On ESP-32, enabling the timer seems to result in a near-synchronous
  interrupt being delivered despite my best attempts to keep it
  masked, then blowing things up because the CPU record isn't set up
  to handle it yet.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>