esp32: Add some "logging" voodoo to SMP initialization

At least one of my WROOM boards has trouble getting the APPCPU
started, that was magically getting better when I started adding
printk's to debug.  It turns out that UART output (and NOT simply idle
cycles of delay) was the magic dust to fix things.  As this SMP
implementation is reverse engineered voodoo to begin with, this hack
should be acceptable in the medium term.

See in-file comments on smp_log() for details.

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