kernel - Allow kprintf() output targets to be managed as a debugging aid.

kernel - Allow kprintf() output targets to be managed as a debugging aid.

* Add kern.kprintf_logging which is a bitmask of kprintf() logging targets.
  This can be adjusted to reduce kprintf() overhead and serialization when
  using it to debug kernel issues.

* The default value is 0x05 which outputs to the console and the
  dmesg buffer.  One can disable console logging and spin-lock
  serialization by setting this to 0x14, causing kprintf() to only
  log to the dmesg buffer.

  0x01	Output to console
  0x02	(unsafe for general kprintf operation, do not use)
  0x04	Output to dmesg buffer
  0x08	(do not use)
  0x10	Do not serialize the whole kprintf() with a spin-lock.